Secondary battery and manufacturing method of the same

ABSTRACT

A negative electrode and a secondary battery including the negative electrode are provided. A plurality of projections and depressions are provided in a negative electrode active material layer and a negative electrode current collector. The plurality of projections and depressions in the negative electrode active material layer absorb expansion of the negative electrode active material and suppress deformation thereof. The plurality of projections and depressions in the negative electrode current collector suppress deformation of the negative electrode current collector caused by expansion and contraction of the negative electrode active material.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. In addition, the present invention relates to a process, amachine, manufacture, or a composition of matter. In particular, oneembodiment of the present invention relates to a semiconductor device, adisplay device, a light-emitting device, a power storage device, animaging device, a driving method thereof, or a manufacturing methodthereof. In particular, one embodiment of the present invention relatesto a negative electrode for a secondary battery and a secondary battery.

2. Description of the Related Art

As portable information terminals typified by smart phones and tabletterminals and mobile devices such as laptop personal computers andportable game machines became popular, secondary batteries having highenergy density which are small and lightweight became urgentlynecessary, and some of them have been developed in recent years.

It is important to increase capacity density of a negative electrode inorder to increase the energy density of a secondary battery. Graphite,which is widely used as a negative electrode active material of alithium-ion secondary battery, has a theoretical density of capacity of372 mAh/g, and already has 360 mAh/g or more capacity density which isclose to the theoretical capacity.

To further increase the energy density of a secondary battery, anegative electrode active material with higher theoretical density ofcapacity has been examined, and silicon (Si), tin (Sn), germanium (Ge),and gallium (Ga) which are alloyed with lithium, an oxide thereof, analloy thereof, and the like have attracted attention.

For example, Patent Document 1 and Patent Document 2 disclose asecondary battery in which a material containing silicon is used for anegative electrode active material.

REFERENCE Patent Document

[Patent Document 1] International Publication WO 2007/074654 Pamphlet

[Patent Document 2] Japanese Published Patent Application No. 2012-38528

SUMMARY OF THE INVENTION

However, the volume of these materials alloyed with lithium, an oxidethereof, an alloy thereof, or the like is greatly changed due tocharging and discharging. For example, the volume of the alloy ofsilicon and lithium increases to be approximately 4 times that ofsilicon.

This causes a problem in that, for example, a negative electrode activematerial is pulverized and separated from a current collector because ofrepeated expansion and contraction, or a negative electrode currentcollector is stretched as the volume of the negative electrode activematerial increases, leading to wrinkles in the negative electrodecurrent collector. The separation of the negative electrode activematerial reduces the capacity of a secondary battery. In addition, whenlarge wrinkles arc generated in the negative electrode currentcollector, the volume of the secondary battery including an exteriorbody is increased, whereby the energy density of the secondary batteryis decreased.

Thus, an object of one embodiment of the present invention is to providea novel negative electrode for a secondary battery and a novel secondarybattery. Specifically, an object of one embodiment of the presentinvention is to provide a negative electrode which can absorb expansionof a negative electrode active material when the negative electrodeactive material is alloyed, and a secondary battery including thenegative electrode.

An object of one embodiment of the present invention is to provide anovel power storage device, a novel negative electrode, an electronicdevice on which a secondary battery including the negative electrode isprovided, or the like. Note that the descriptions of these objects donot disturb the existence of other objects. In one embodiment of thepresent invention, there is no need to achieve all the objects. Otherobjects will be apparent from and can be derived from the description ofthe specification, the drawings, the claims, and the like.

In order to achieve any of the above objects, in one embodiment of thepresent invention, a plurality of projections and depressions areprovided in a negative electrode active material layer and a negativeelectrode current collector. The plurality of projections anddepressions provided in the negative electrode active material layerabsorb expansion of the negative electrode active material and suppressdeformation of the negative electrode active material layer. Inaddition, a plurality of projections and depressions provided in thenegative electrode current collector suppress deformation of thenegative electrode current collector caused by expansion and contractionof the negative electrode active material.

One embodiment of the present invention is a secondary battery includinga negative electrode, a positive electrode, an electrolyte solution, anda separator. The negative electrode includes a negative electrodecurrent collector and a negative electrode active material layer. Thenegative electrode active material layer includes a compound containingsilicon as a negative electrode active material. The negative electrodeactive material layer has a first pair of a projection and a depressionand a second pair of a projection and a depression. The negativeelectrode current collector has a third pair of a projection and adepression and a fourth pair of a projection and a depression. The firstpair of the projection and the depression of the negative electrodeactive material layer and the third pair of the projection and thedepression of the negative electrode current collector overlap with eachother. The second pair of the projection and the depression of thenegative electrode active material layer and the fourth pair of theprojection and the depression of the negative electrode currentcollector overlap with each other.

According to one embodiment of the present invention, a novel negativeelectrode for a secondary battery and a secondary battery can beprovided. Specifically, a negative electrode which can absorb expansionof a negative electrode active material when the negative electrodeactive material is alloyed, and a secondary battery including thenegative electrode can be provided.

Furthermore, a novel power storage device, a novel negative electrodefor a secondary battery, an electronic device including a secondarybattery, or the like can be provided. Note that the description of theseeffects does not disturb the existence of other effects. One embodimentof the present invention does not necessarily achieve all the objectslisted above. Other effects will be apparent from and can be derivedfrom the description of the specification, the drawings, the claims, andthe like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top view of a negative electrode which can be used in asecondary battery and FIGS. 1B and 1C are cross-sectional views thereof.

FIGS. 2A to 2D are cross-sectional views of a negative electrode whichcan be used in a secondary battery.

FIGS. 3A1, 3A2, 3A3, 3B1, and 3B2 are top views of a negative electrodewhich can be used in a secondary battery.

FIG. 4 is a perspective view illustrating an example of a secondarybattery.

FIGS. 5A and 5B illustrate cross-sectional views of an example of asecondary battery.

FIGS. 6A to 6D illustrate an example of a manufacturing method of asecondary battery.

FIGS. 7A and 7B illustrate an example of a manufacturing method of asecondary battery.

FIGS. 8A to 8C illustrate an example of a manufacturing method of asecondary battery.

FIGS. 9A and 9B illustrate an example of a manufacturing method of asecondary battery.

FIGS. 10A and 10B illustrate an example of a manufacturing method of asecondary battery.

FIGS. 11A and 11B illustrate an example of a secondary battery.

FIGS. 12A to 12C illustrate an example of secondary battery.

FIGS. 13A to 13C illustrate an example of a secondary battery.

FIGS. 14A and 14B illustrate an example of a power storage system.

FIGS. 15A1, 15A2, 15B1, and 15B2 illustrate an example of a powerstorage system.

FIGS. 16A and 16B illustrate an example of a power storage system.

FIG. 17 is a block diagram illustrating a battery management unit of apower storage device.

FIGS. 18A to 18C are conceptual diagrams illustrating a batterymanagement unit of a power storage device.

FIG. 19 is a circuit diagram illustrating a battery management unit of apower storage device.

FIG. 20 is a circuit diagram illustrating a battery management unit of apower storage device.

FIGS. 21A to 21C are conceptual diagrams illustrating a batterymanagement unit of a power storage device.

FIG. 22 is a block diagram illustrating a battery management unit of apower storage device.

FIG. 23 is a flow chart showing an operation of a battery managementunit of a power storage device.

FIGS. 24A to 24F illustrate examples of electronic devices.

FIGS. 25A to 25C illustrate an example of an electronic device.

FIG. 26 illustrates examples of electronic devices.

FIG. 27 illustrates examples of electronic devices.

FIGS. 28A and 28B illustrate examples of electronic devices.

FIGS. 29A and 29B are photographs of a negative electrode before beingcharged and discharged.

FIGS. 30A to 30C are X-ray CT images of negative electrodes aftercharging and discharging.

FIGS. 31A to 31C are X-ray CT images of negative electrodes aftercharging and discharging.

FIG. 32 is a graph showing charge and discharge characteristics of asecondary battery.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings. However, the present invention is notlimited to the description below, and it is easily understood by thoseskilled in the art that modes and details disclosed herein can bemodified in various ways. Further, the present invention is notconstrued as being limited to description of the embodiments and theexamples.

The term “electrically connected” includes the case where components areconnected through an “object having any electric function”. There is noparticular limitation on the “object having any electric function” aslong as electric signals can be transmitted and received between thecomponents connected through the object.

Note that the position, size, length, range, or the like of eachstructure illustrated in the drawings and the like is not accuratelyrepresented in some cases for easy understanding. Therefore, thedisclosed invention is not necessarily limited to the position, thesize, the length, the range, or the like disclosed in the drawings andthe like.

The ordinal number such as “first”, “second”, and “third” are used toavoid confusion among components.

EMBODIMENT 1

In this embodiment, a negative electrode which can be used in asecondary battery of one embodiment of the present invention isdescribed with reference to FIGS. 1A to 1C, FIGS. 2A to 2D, and FIGS.3A1, 3A2, 3A3, 3B1, and 3B2.

FIG. 1A is a top view of a negative electrode 111 which can be used inthe secondary battery of one embodiment of the present invention, andFIG. 1B is a cross-sectional view taken along line X1-X2 in FIG. 1A, andFIG. 1C is a cross-sectional view taken along line Y1 -Y2 in FIG. 1A.

The negative electrode 111 includes a negative electrode currentcollector 101 and a negative electrode active material layer 102 formedover the negative electrode current collector 101. The negativeelectrode active material layer 102 includes a negative electrode activematerial. The negative electrode active material layer 102 may furtherinclude a binder for increasing adhesion of negative electrode activematerials, a conductive additive for increasing the conductivity of thenegative electrode active material layer, and the like.

There is no particular limitation on the materials used for the negativeelectrode current collector 101 as long as it has high conductivitywithout causing a significant chemical change in the secondary battery.For example, the current collector can be formed using a metal such asgold, platinum, iron, nickel, copper, aluminum, titanium, tantalum, ormanganese, or an alloy thereof (e.g., stainless steel). Furthermore,coating with carbon, nickel, titanium, or the like may be performed.Silicon, neodymium, scandium, molybdenum, or the like may be added toimprove heat resistance. The current collector can each have any ofvarious shapes including a foil-like shape, a sheet-like shape, aplate-like shape, a net-like shape, a cylindrical shape, a coil shape, apunching-metal shape, an expanded-metal shape, a porous shape, and ashape of non-woven fabric as appropriate. The current collector may beformed to have micro irregularities on the surface thereof in order toenhance adhesion to the active material. The current collectorpreferably has a thickness of more than or equal to 5 μm and less thanor equal to 30 μm.

As a negative electrode active material included in the negativeelectrode active material layer 102, a material which enablescharge-discharge reaction by alloying and dealloying reaction withlithium ion can be used. Typically, silicon (Si), tin (Sn), zinc (Zn),germanium (Ge), gallium (Ga), an oxide thereof, an alloy thereof, andthe like can be given. The use of a material which can be alloyed withlithium, an oxide thereof, an alloy thereof, and the like enables thesecondary battery with higher capacity than that in the case of using acarbon-based negative electrode material. Therefore, it is particularlypreferable to use a material containing silicon such as SiO as anegative electrode active material.

Note that SiO refers to the powder of a silicon oxide including asilicon-rich portion and can also be referred to as SiO_(y) (2>y>0).Examples of SiO include a material containing one or more of Si₂O₃,Si₃O₄, and Si₂O and a mixture of Si powder and silicon dioxide (SiO₂).In particular, Si particles with a particle size of less than or equalto 10 pm are preferably dispersed in each of SiO particles. When smallSi particles are dispersed in each of SiO particles, a crack of SiOparticles caused by expansion and contraction of Si can be suppressed.Here, a composition of the SiO particle is preferably within a range ofSiO_(y) (0.95≤y≤1.05). Furthermore, SiO may contain another element(e.g., carbon, nitrogen, iron, aluminum, copper, titanium, calcium, andmanganese). In other words, SiO refers to a colored material containingtwo or more of single crystal silicon, amorphous silicon, polycrystalsilicon, Si₂O₃, Si₃O₄, Si₂O, and SiO₂. Thus, SiO can be distinguishedfrom SiO_(x)(x is 2 or more), which is clear and colorless or white.Note that in the case where a secondary battery is fabricated using SiOas a material thereof and the SiO is oxidized because of repeated chargeand discharge cycles, SiO is changed into SiO₂ in some cases.

As another example of a material which enables charge-discharge reactionby alloying and dealloying reaction with lithium ion, a materialcontaining at least one of Mg, Ca, Al, Ge, Sn, Pb, As, Sb, Bi, Ag, Au,Zn, Cd, Hg, In, and the like can be used. Examples of an alloy-basedmaterial using such elements include Mg₂Si, Mg₂Ge, SnO, SnO₂, Mg₂Sn,SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb, CeSb₃,LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, SbSn, and the like.

As a binder which can be used in the negative electrode active materiallayer 102, polyimide, PVDF, polytetrafluoroethylene, polyvinyl chloride,ethylene-propylene-diene polymer, styrene-butadiene rubber,acrylonitrile-butadiene rubber, fluorine rubber, polyvinyl acetate,polymethyl methacrylate, polyethylene, nitrocellulose, or the like isgiven. Polyimide is particularly preferable because polyimide canwithstand expansion and contraction of the negative electrode activematerial due to charging and discharging.

As a conductive additive which can be used in the negative electrodeactive material layer 102, acetylene black, graphene, graphite, grapheneoxide, graphite oxide, a carbon nanotube, fullerene or the like isgiven.

The negative electrode current collector 101 and the negative electrodeactive material layer 102 each have a plurality of projections anddepressions. In addition, the projections and depressions of thenegative electrode current collector 101 and those of the negativeelectrode active material layer 102 overlap with each other. In FIGS. 1Ato 1C, the negative electrode current collector 101 and the negativeelectrode active material layer 102 have a plurality of lineardepressions which are parallel to each other.

As illustrated in FIGS. 1B and 1C, the depression of the negativeelectrode active material layer 102 may have a shallower depth than thethickness of the negative electrode active material layer 102, or may bedeep enough to expose the negative electrode current collector 101. Inother words, the negative electrode current collector 101 may be exposedat part of the bottoms of the depressions of the negative electrodeactive material layer 102. Furthermore, the depressed portions withdifferent depths may be formed. As illustrated in FIG. 1A, the depressedportions may be provided with different intervals.

The depressions in the negative electrode active material layer 102 canabsorb expansion of the negative electrode active material and suppressthe deformation of the negative electrode active material layer 102 evenwhen a negative electrode active material having a large volume changecaused by charging and discharging is used. Thus, it is possible tosuppress separation of the negative electrode active material from thecurrent collector due to pulverization caused by repetitive volumeexpansion and contraction, and improve the cycle characteristics of thesecondary battery. Since wrinkles in the negative electrode currentcollector 101, which are generated when the negative electrode currentcollector 101 is stretched as the volume of the negative electrodeactive material increases, can be suppressed, the volume increase of thenegative electrode can be suppressed, so that a decrease in energydensity of the secondary battery can be prevented.

In addition, depressions in the negative electrode current collector 101can suppress deformation of the negative electrode current collectorcaused by expansion and contraction of the negative electrode activematerial.

The region where projections and depressions are formed in the negativeelectrode current collector 101 and the negative electrode activematerial layer 102 is preferably a region overlapping with a positiveelectrode when incorporated in the secondary battery. This is because anegative electrode active material largely expands and contracts in itsregion overlapping with the positive electrode. Therefore, it ispreferable that a length L2 of the depression in the negative electrodecurrent collector 101 and the negative electrode active material layer102 illustrated in FIG. 1A be approximately the same as a width of theoverlapping positive electrode.

It is effective to make the area of the negative electrode 111 largerthan that of the positive electrode in order to suppress lithiumdeposition on the surface of the negative electrode 111 caused bycharging and discharging of the secondary battery. However, in the casewhere the difference in area between the positive electrode and thenegative electrode 111 is too large, the energy density of the secondarybattery is decreased. Therefore, for example, the length L2 of thedepression is preferably 80% or more and 100% or less of a length L1 ofa side of the negative electrode current collector 101 which is parallelto the depression, more preferably 85% or more and 98% or less of thelength L1.

A depth H2 of a depression of the negative electrode active materiallayer 102 is preferably as deep as possible, in which case expansion ofthe negative electrode active material can be absorbed more easily.Therefore, H2 is preferably 90% or more and 100% or less of a thicknessH1 of the negative electrode active material layer 102, for example.

A smaller width W1 between adjacent depressions in the negativeelectrode current collector 101 and the negative electrode activematerial layer 102 is advantageous in absorbing expansion of thenegative electrode active material; however, when W1 is too small, thenegative electrode capacity is decreased and energy density of thesecondary battery is decreased. Therefore, W1 is preferably greater thanor equal to 0.1 mm and less than or equal to 5 mm, more preferablygreater than or equal to 0.5 mm and less than or equal to 2 mm, forexample.

A larger width W2 of a depression in the negative electrode currentcollector 101 and the negative electrode active material layer 102 isadvantageous in absorbing the expansion of the negative electrode activematerial; however, when W2 is too large, the negative electrode capacityis decreased and energy density of the secondary battery is decreased.Therefore, W2 is preferably less than or equal to W1. W2 is preferablygreater than or equal to 0.1 mm and less than or equal to 1 mm, morepreferably greater than or equal to 0.25 mm and less than or equal to0.45 mm.

The shape of the projections and depressions of the negative electrodeactive material layer 102 and the negative electrode current collector101 is not limited to that illustrated in FIGS. 1A to 1C. For example,the depression may have a shape close to a triangular prism asillustrated in FIG. 2A. Alternatively, for example, the depression mayhave a shape close to a quadrangular prism as illustrated in FIG. 2B.

Furthermore, although the negative electrode active material layer 102is formed on one surface of the negative electrode current collector 101in FIGS. 1A to 1C and FIGS. 2A and 2B, the negative electrode activematerial layer 102 may be formed on both surfaces of the negativeelectrode current collector 101 as illustrated in FIGS. 2C and 2D.Forming the negative electrode active material layer 102 on bothsurfaces of the negative electrode current collector 101 increases thecapacity of the secondary battery.

In the case where the negative electrode active material layer 102 isformed on both surfaces of the negative electrode current collector 101,it is preferable that the positions of the depressions in one surface ofthe negative electrode active material layer 102 do not align with thoseof the depressions in the other surface of the negative electrode activematerial layer 102 as illustrated in FIGS. 2C and 2D. This is becausethe strength of the negative electrode current collector 101 might bereduced when positions of the depressions on the both surfaces alignwith each other.

Furthermore, the pattern of forming projections and depressions is notlimited to those illustrated in FIGS. 1A and 1B and FIGS. 2A to 2D. Forexample, as illustrated in FIG. 3A1, the uneven pattern of the negativeelectrode active material layer 102 and the negative electrode currentcollector 101 may be formed parallel to the long side of the negativeelectrode current collector 101. Alternatively, as illustrated in FIG.3A2, the uneven pattern may be a lattice pattern. Alternatively, asillustrated in FIG. 3A3, the uneven pattern may be a pattern that isformed by a series of continuing hexagonal shapes. Alternatively, asillustrated in FIG. 3B1, the uneven pattern may be a pattern that isformed by a series of continuing triangles. Alternatively, asillustrated in FIG. 3B2, the uneven pattern may be a pattern ofconcentric circles.

In Embodiment 1, one embodiment of the present invention has beendescribed. Other embodiments of the present invention are described inEmbodiments 2 to 8. Note that one embodiment of the present invention isnot limited to the above examples. That is, since various embodiments ofthe present invention are disclosed in Embodiment 1 and. Embodiments 2to 8, one embodiment of the present invention is not limited to aspecific embodiment. The example in which one embodiment of the presentinvention is applied to a lithium-ion secondary battery is described;however, one embodiment of the present invention is not limited thereto.Depending on circumstances or conditions, one embodiment of the presentinvention may be applied to a variety of secondary batteries such as alead storage battery, a lithium-ion polymer secondary battery, anickel-hydrogen storage battery, a nickel-cadmium storage battery, anickel-iron storage battery, a nickel-zinc storage battery, a silveroxide-zinc storage battery, a solid-state battery, and an air battery, aprimary battery, a capacitor, a lithium ion capacitor, and the like.Alternatively, for example, depending on circumstances or conditions,one embodiment of the present invention is not necessarily applied to alithium-ion secondary battery. For example, the example in which thenegative electrode current collector or the negative electrode activematerial has projections and depressions is described; however, oneembodiment of the present invention is not limited to thereto.Alternatively, for example, depending on circumstances or conditions, amaterial other than the negative electrode current collector or thenegative electrode current material layer may have projections anddepressions. Alternatively, for example, depending on circumstances orconditions, the negative electrode current collector or the negativeelectrode current material layer may have a shape other than projectionsand depressions in one embodiment of the present invention.Alternatively, for example, depending on circumstances or conditions,the negative electrode current collector and the negative electrodeactive material layer do not necessarily have projections anddepressions in one embodiment of the present invention.

EMBODIMENT 2

In one embodiment of the present invention, a specific structure and aspecific material of the secondary battery of one embodiment of thepresent invention arc described with reference to FIG. 4 and FIGS. 5Aand 5B. In this embodiment, an example where one of a positive electrodeand a negative electrode is covered with a bag-shaped separator will bedescribed below.

FIG. 4 is a perspective view showing an appearance of the secondarybattery 200. FIG. 5A is a cross-sectional view of a portion indicated bythe dashed-dotted line A1-A2 in FIG. 4. FIG. 5B is a cross-sectionalview of a portion along dashed-dotted line B1-B2 in FIG. 4.

The secondary battery 200 of one embodiment of the present inventionincludes a positive electrode 215 covered with a separator 203, thenegative electrode 111, and an electrolyte solution 204 in an exteriorbody 207. In this embodiment, an example of a secondary batteryincluding two positive electrode current collectors each having apositive electrode active material layer on both surfaces, two negativeelectrode current collectors each having a negative electrode activematerial layer on one surface, and one negative electrode currentcollector having a negative electrode active material layer on bothsurfaces. The positive electrode 215 is electrically connected to apositive electrode lead 221. The negative electrode 111 is electricallyconnected to a negative electrode lead 225. Each of the positiveelectrode lead 221 and the negative electrode lead 225 is also referredto as a lead electrode or a lead terminal. Parts of the positiveelectrode lead 221 and the negative electrode lead 225 are positionedoutside the exterior body. The secondary battery 200 is charged anddischarged through the positive electrode lead 221 and the negativeelectrode lead 225.

Although FIGS. 5A and 5B illustrate the example in which the positiveelectrode 215 is covered with the separator 203, but one embodiment ofthe present invention is not limited thereto. For example, the positiveelectrode 215 is not necessarily covered with the separator 203; insteadof the positive electrode 215, the negative electrode 111 may be coveredwith the separator 203.

[1. Positive Electrode]

The positive electrode 215 includes the positive electrode currentcollector 205, the positive electrode active material layer 206 formedon the positive electrode current collector 205, and the like. FIG. 5illustrates an example where the positive electrode active materiallayer 206 is formed on both surfaces of the positive electrode currentcollector 205 with a sheet shape (or a strip-like shape); however, thepositive electrode active material layer 206 may be formed on onesurface of the positive electrode current collector 205.

The positive electrode current collector 205 can be formed using amaterial that has high conductivity and does not cause a significantchemical change, such as a metal typified by stainless steel, gold,platinum, aluminum, or titanium, or an alloy thereof. Alternatively, analuminum alloy to which an element which improves heat resistance, suchas silicon, titanium, neodymium, scandium, or molybdenum, is added canbe used. Still alternatively, a metal element which forms silicide byreacting with silicon can be used. Examples of the metal element whichforms silicide by reacting with silicon include zirconium, titanium,hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten,cobalt, nickel, and the like. The positive electrode current collector205 can have a foil-like shape, a plate-like shape (a sheet-like shape),a net-like shape, a punching-metal shape, an expanded-metal shape, orthe like as appropriate. The positive electrode current collector 205preferably has a thickness greater than or equal to 5 μm and less thanor equal to 30 μm. The surface of the positive electrode currentcollector 205 may be provided with an undercoat using graphite or thelike.

The positive electrode active material layer 206 may further include abinder for increasing adhesion of positive electrode active materials, aconductive additive for increasing the conductivity of the positiveelectrode active material layer 206, and the like in addition to thepositive electrode active materials.

Examples of a positive electrode active material used for the positiveelectrode active material layer 206 include a composite oxide with anolivine crystal structure, a composite oxide with a layered rock-saltcrystal structure, and a composite oxide with a spinel crystalstructure. As the positive electrode active material, a compound such asLiFeO₂, LiCoO₂, LiNiO₂, LiMn₂O₄, V₂O₅, Cr₂O₅, and MnO₂ is used.

LiCoO₂ is particularly preferable because it has high capacity,stability in the air higher than that of LiNiO₂, and thermal stabilityhigher than that of LiNiO₂, for example.

It is preferable to add a small amount of lithium nickel oxide (LiNiO₂or LiNi_(1-x)M_(x)O₂ (M=Co, Al, or the like)) to a lithium-containingmaterial with a spinel crystal structure which contains manganese suchas LiMn₂O₄ because characteristics of the secondary battery using such amaterial can be improved. Typically, a material represented by acomposition formula Li_(1.68)Mn_(0.8062)Ni_(0.318)O₃ can be given.

Alternatively, a complex material (LiMPO₄ (general formula) (M is one ormore of Fe(II), Mn(II), Co(II), and Ni(II))) can be used. Typicalexamples of the general formula LiMPO₄ which can be used as a materialare lithium compounds such as LiFePO₄, LiNiPO₄, LiCoPO₄, LiMnPO₄,LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄,LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≤1, 0<a<1, and 0<b<1),LiFe_(c)Ni_(d)Co_(e)PO₄, LiFe_(c)Ni_(d)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≤1, 0<c<1, 0<d<1, and 0<e<1), andLiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≤1, 0<f<1, 0<g<1, 0<h<1, and0<i<1).

LiFePO₄ is particularly preferable because it meets requirements withbalance for a positive electrode active material, such as safety,stability, high capacity density, and the existence of lithium ions thatcan be extracted in initial oxidation (charging).

Alternatively, a complex material such as Li(_(2-j))MSiO₄ (generalformula) (M is one or more of Fe(II), Mn(II), Co(II), and Ni(II); 0≤j≤2)may be used. Typical examples of the general formula Li(_(2-j))MSiO₄which can be used as a material are lithium compounds such asLi(_(2-j))FeSiO₄, Li(_(2-j))NiSiO₄, Li(_(2-j))CoSiO₄, Li(_(2-j))MnSiO₄,Li(_(2-j))Fe_(k)Ni_(l)SiO₄, Li(_(2-j))Fe_(k)Co_(l)SiO₄,Li(_(2-j))Fe_(k)Mn_(l)SiO₄, Li(_(2-j))Ni_(k)Co_(l)SiO₄,Li(_(2-j))Ni_(k)Mn_(l)SiO₄ (k+l≤1, 0<k<1, and 0<l<1),Li(_(2-j))Fe_(m)Ni_(n)Co_(q)SiO₄, Li(_(2-j))Fe_(m)Ni_(n)Mn_(q)SiO₄,Li(_(2-j))Ni_(m)Co_(n)Mn_(q)SiO₄ (m+n+q≤1, 0<m<1, 0<n<1, and 0<q<1), andLi(_(2-j))Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄ (r+s+t+u≤1, 0<r<1, 0<s<1, 0<t<1,and 0<u<1).

Still alternatively, a nasicon compound expressed by A_(x)M₂(XO₄)₃(general formula) (A=Li, Na, or Mg, M=Fe, Mn, Ti, V, or Nb, X=S, P, Mo,W, As, or Si) can be used for the positive electrode active material.Examples of the nasicon compound are Fe₂(MnO₄)₃, Fe₂(SO₄)₃, andLi₃Fe₂(PO₄)₃. Further alternatively, a compound expressed by Li₂MPO₄F,Li₂MP₂O₇, or Li₅MO₄ (general formula) (M=Fe or Mn), a perovskitefluoride such as NaFeF₃ and FeF₃, a metal chalcogenide (a sulfide, aselenide, or a telluride) such as TiS₂ and MoS₂, an oxide with aninverse spinel structure such as LiMVO₄ (M=Mn, Co, Ni), a vanadium oxide(V₂O₅, V₆O₁₃, LiV₃O₈, or the like), a manganese oxide, an organic sulfurcompound, or the like can be used as the positive electrode activematerial.

In the case where carrier ions are alkali metal ions other than lithiumions or alkaline-earth metal ions, the positive electrode activematerial may contain, instead of lithium, an alkali metal (e.g., sodiumor potassium) or an alkaline-earth metal (e.g., calcium, strontium,barium, beryllium, or magnesium). For example, the positive electrodeactive material may be a layered oxide containing sodium such as NaFeO₂or Na_(2/3)[Fe_(1/2)Mn_(1/2)]O₂.

Further alternatively, any of the aforementioned materials may becombined to be used as the positive electrode active material. Forexample, a solid solution obtained by combining two or more of the abovematerials can be used as the positive electrode active material. Forexample, a solid solution of LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂ and Li₂MnO₃can be used as the positive electrode active material.

Note that although not shown, a conductive material such as a carbonlayer may be provided on a surface of the positive electrode activematerial layer 206. With the conductive material such as the carbonlayer, conductivity of the electrode can be increased. For example, thepositive electrode active material layer 206 can be coated with a carbonlayer by mixing a carbohydrate such as glucose at the time of baking thepositive electrode active material.

The average particle diameter of the primary particle of the positiveelectrode active material layer 206 is preferably greater than or equalto 50 nm and less than or equal to 100 μm.

Examples of the conductive additive include acetylene black (AB),graphite (black lead) particles, carbon nanotubes, graphenc, andfullerene.

A network for electric conduction can be formed in the positiveelectrode 215 by the conductive additive. The conductive additive alsoallows maintaining of a path for electric conduction between theparticles of the positive electrode active material layer 206. Theaddition of the conductive additive to the positive electrode activematerial layer 206 increases the electric conductivity of the positiveelectrode active material layer 206.

As the binder, instead of polyvinylidene fluoride (PVDF) as a typicalone, polyimide, polytetrafluoroethylene, polyvinyl chloride,ethylene-propylene-diene polymer, styrene-butadiene rubber,acrylonitrile-butadiene rubber, fluorine rubber, polyvinyl acetate,polymethyl methacrylate, polyethylene, nitrocellulose or the like can beused.

The content of the binder in the positive electrode active materiallayer 206 is preferably greater than or equal to 1 wt % and less than orequal to 10 wt %, more preferably greater than or equal to 2 wt % andless than or equal to 8 wt %, and still more preferably greater than orequal to 3 wt % and less than or equal to 5 wt %. The content of theconductive additive in the positive electrode active material layer 206is preferably greater than or equal to 1 wt % and less than or equal to10 wt %, more preferably greater than or equal to 1 wt % and less thanor equal to 5 wt %.

In the case where the positive electrode active material layer 206 isformed by a coating method, the positive electrode active material, thebinder, and the conductive additive are mixed to form a positiveelectrode paste (slurry), and the positive electrode paste is applied tothe positive electrode current collector 205 and dried.

[2. Negative Electrode]

The negative electrode described in Embodiment 1 can be used as thenegative electrode 111.

Graphene may be formed on a surface of the negative electrode activematerial layer 102. In the case of using silicon as the negativeelectrode active material, the volume of silicon is greatly changed dueto occlusion and release of carrier ions in charge-discharge cycles.Therefore, adhesion between the negative electrode current collector 101and the negative electrode active material layer 102 is decreased,resulting in degradation of battery characteristics caused by charge anddischarge. Thus, graphene is preferably formed on a surface of thenegative electrode active material layer 102 containing silicon becauseeven when the volume of silicon is changed in charge-discharge cycles,decrease in the adhesion between the negative electrode currentcollector 101 and the negative electrode active material layer 102 canbe inhibited, which makes it possible to reduce degradation of batterycharacteristics.

Further, a coating film of oxide or the like may be formed on thesurface of the negative electrode active material layer 102. A coatingfilm formed by decomposition or the like of an electrolyte solution orthe like in charging cannot release electric charges used at theformation, and therefore forms irreversible capacity. In contrast, thefilm of an oxide or the like provided on the surface of the negativeelectrode active material layer 102 in advance can reduce or preventgeneration of irreversible capacity.

As the coating film coating the negative electrode active material layer102, an oxide film of any one of niobium, titanium, vanadium, tantalum,tungsten, zirconium, molybdenum, hafnium, chromium, aluminum, andsilicon or an oxide film containing any one of these elements andlithium can be used. Such a film is denser than a conventional filmformed on a surface of a negative electrode due to a decompositionproduct of an electrolyte solution.

For example, niobium oxide (Nb₂O₅) has a low electric conductivity of 10⁻⁹ S/cm and a high insulating property. For this reason, a niobium oxidefilm inhibits electrochemical decomposition reaction between thenegative electrode active material and the electrolyte solution. On theother hand, niobium oxide has a lithium diffusion coefficient of 10 ⁻⁹cm²/sec and high lithium ion conductivity. Therefore, niobium oxide cantransmit lithium ions. Alternatively, silicon oxide or aluminum oxidemay be used.

A sol-gel method can be used to coat the negative electrode activematerial layer 102 with the coating film, for example. The sol-gelmethod is a method for forming a thin film in such a manner that asolution of metal alkoxide, a metal salt, or the like is changed into agel, which has lost its fluidity, by hydrolysis reaction andpolycondensation reaction and the gel is baked. Since a thin film isfoiined from a liquid phase in the sol-gel method, raw materials can bemixed uniformly on the molecular scale. For this reason, by adding anegative electrode active material such as graphite to a raw material ofthe metal oxide film which is a solvent, the active material can beeasily dispersed into the gel. In such a manner, the coating film can beformed on the surface of the negative electrode active material layer102. A decrease in the capacity of the power storage unit can beprevented by using the coating film.

[3. Separator]

As a material of the separator 203, a porous insulator such ascellulose, polypropylene (PP), polyethylene (PE), polybutene, nylon,polyester, polysulfone, polyacrylonitrile, polyvinylidene fluoride, ortetrafluoroethylene can be used. Alternatively, nonwoven fabric of aglass fiber or the like, or a diaphragm in which a glass fiber and apolymer fiber are mixed may be used.

[4. Electrolyte Solution]

As a solvent of an electrolyte solution 204 used for the secondarybattery 200, an aprotic organic solvent is preferable. For example, oneof ethylene carbonate (EC), propylene carbonate (PC), butylenecarbonate, chloroethylene carbonate, vinylene carbonate,γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethylcarbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methylacetate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane(DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile,benzonitrile, tetrahydrofuran, sulfolane, and sultone can be used, ortwo or more of these solvents can be used in an appropriate combinationin an appropriate ratio.

When a gelled high-molecular material is used as the solvent for theelectrolyte solution, safety against liquid leakage and the like isimproved. Further, a secondary battery can be thinner and morelightweight. Typical examples of gelled high-molecular materials includea silicone gel, an acrylic gel, an acrylonitrile gel, a polyethyleneoxide-based gel, a polypropylene oxide-based gel, a fluorine-basedpolymer gel, and the like.

Alternatively, the use of one or more ionic liquids (room temperaturemolten salts) which are less likely to burn and volatilize as thesolvent for the electrolyte solution can prevent the power storage unitfrom exploding or catching fire even when the secondary batteryinternally shorts out or the internal temperature increases due toovercharging or the like.

In the case of using a lithium ion as a carrier ion, as an electrolytedissolved in the above-described solvent, one of lithium salts such asLiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiAlCl₄, LiSCN, LiBr, LiI, Li₂SO₄,Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃, LiC(CF₃SO₂)₃,LiC(C₂F₅SO₂)₃, LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂) (CF₃SO₂), and LiN(C₂F₅SO₂)₂can be used, or two or more of these lithium salts can be used in anappropriate combination in an appropriate ratio.

The electrolyte solution used for the secondary battery preferablycontains a small amount of dust particles and elements other than theconstituent elements of the electrolyte solution (hereinafter, alsosimply referred to as impurities) so as to be highly purified.Specifically, the weight ratio of impurities to the electrolyte solutionis less than or equal to 1%, preferably less than or equal to 0.1%, andmore preferably less than or equal to 0.01%. An additive agent such asvinylene carbonate may be added to the electrolyte solution.

[5. Exterior Body]

There are a variety of structures of the exterior body of the secondarybattery, and a film is used for formation of the exterior body 207 inthis embodiment. In this embodiment, a film is used for the exteriorbody 207. Note that the film used for the exterior body 207 is asingle-layer film selected from a metal film (e.g., an aluminum film, astainless steel film, and a nickel steel film), a plastic film made ofan organic material, a hybrid material film including an organicmaterial (e.g., an organic resin or fiber) and an inorganic material(e.g., ceramic), and a carbon-containing inorganic film (e.g., a carbonfilm or a graphite film); or a stacked-layer film including two or moreof the above films. Forming depressions or projections by embossingincreases the surface area of the exterior body 207 exposed to outsideair, achieving efficient heat dissipation.

In the case where the secondary battery 200 is changed in form byexternally applying force, bending stress is externally applied to theexterior body 207 of the secondary battery 200. This might partly deformor damage the exterior body 207. Projections or depressions formed onthe exterior body 207 can relieve a strain caused by stress applied tothe exterior body 207. Thus, the secondary battery 200 can have higherreliability. Note that a “strain” is the scale of change in formindicating the displacement of a point of an object relative to thereference (initial) length of the object. By forming projections ordepressions on the exterior body 207, the influence of a strain causedby externally applying force to the power storage unit can be reduced tobe acceptable. Thus, the power storage unit having high reliability canbe provided.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and example.

EMBODIMENT 3

In this embodiment, another example of a method for manufacturing thesecondary battery 200 will be described with reference to FIGS. 6A to6D, FIGS. 7A and 7B, FIGS. 8A to 8C, and FIGS. 9A and 9B.

[1. Covering Positive Electrode with Separator]

First, the positive electrode 215 in which the positive electrode activematerial layer 206 is formed is positioned over the separator 203 (seeFIG. 6A). Next, the separator 203 is folded along a dotted line in FIG.6A (see FIG. 6B) so that the positive electrode 215 is interposedbetween the surfaces of the separator 203 (see FIG. 6C).

Then, the outer edges of the separator 203 outside the positiveelectrode 215 are bonded to form the bag-like separator 203 (see FIG.6D). The bonding of the outer edges of the separator 203 can beperformed with the use of an adhesive or the like, by ultrasonicwelding, or by thermal fusion bonding.

In this embodiment, polypropylene is used as the separator 203, and theouter edges of the separator 203 are bonded by heating. A bondingportion 203 a is shown in FIG. 6D. In this manner, the positiveelectrode 215 can be covered with the separator 203. The separator 203is formed so as to cover the positive electrode active material layer206 and does not necessarily cover the whole positive electrode 215.

Note that although the separator 203 is folded in FIGS. 6A to 6D, oneembodiment of the present invention is not limited thereto. For example,the positive electrode 215 may be interposed between two separators. Inthat case, the bonding portion 203 a may be formed to surround almostall of four sides of the separators.

The outer edges of the separator 203 may be bonded, using dashedline-like or dot-like bonding portions provided at regular intervals.

Alternatively, bonding may be performed on only one side of the outeredges. Alternatively, bonding may be performed on only two sides of theouter edges. Alternatively, bonding may be performed on four sides ofthe outer edges. Accordingly, the four sides can be in an even state.

Note that although the case where the positive electrode 215 is coveredwith the separator 203 is shown in FIGS. 6A to 6D and the like, oneembodiment of the present invention is not limited thereto. For example,the positive electrode 215 is not necessarily covered with the separator203; instead of the positive electrode 215, the negative electrode 111may be covered with the separator 203.

In this embodiment, two positive electrodes in each of which thepositive electrode active material layer 206 is formed on both surfacesof the positive electrode current collector are used.

[2. Preparing Negative Electrode]

Next, the negative electrode 111 in which projections and depressionsare formed on a negative electrode current collector and a negativeelectrode active material layer is prepared. In this embodiment, twonegative electrode current collectors each having a negative electrodeactive material layer on one surface and one negative electrode currentcollector having a negative electrode active material layer on bothsurfaces are used.

[3. Connecting Leads to Positive Electrode and Negative Electrode]

Next, the positive electrode 215 and the negative electrode 111 arestacked with each other as illustrated in FIG. 7A.

Next, the positive electrode lead 221 including the sealing layer 220 iselectrically connected to positive electrode tabs of the plurality ofpositive electrode current collectors 205 by ultrasonic wave irradiationwhile pressure is applied (ultrasonic welding).

The lead electrode is likely to be cracked or cut by stress due toexternal force applied after manufacture of the power storage unit.

Here, when subjected to ultrasonic welding, the positive electrode lead221 is placed between bonding dies provided with projections, whereby aconnection region 222 and a bent portion 223 can be formed in thepositive electrode tab (see FIG. 7B).

This bent portion 223 can relieve stress due to external force appliedafter fabrication of the secondary battery 200. Accordingly, thereliability of the secondary battery 200 can be increased.

The bent portion 223 is not necessarily formed in the positive electrodetab. The positive electrode current collector may be formed using ahigh-strength material such as stainless steel to a thickness of 10 μmor less, in order to easily relieve stress due to external force appliedafter fabrication of a secondary battery.

It is needless to say that two or more of the above examples may becombined to relieve concentration of stress in the positive electrodetab.

Then, in a manner similar to that of the positive electrode currentcollector 205, the negative electrode lead 225 including the sealinglayer 220 is electrically connected to the negative electrode tab of thenegative electrode current collector 101 by ultrasonic welding.

[4. Bonding at Side of Exterior Body]

Next, a film used as an exterior body is folded along a dotted line (seeFIG. 8A), and thermocompression bonding is performed along one side ofthe folded exterior body. A portion where thermocompression bonding isperformed along one side of the exterior body 207 is shown as a bondingportion 207 a in FIG. 8B.

[5. Sealing]

Then, the positive electrode 215 electrically connected to the positiveelectrode lead 221 and the negative electrode 111 electrically connectedto the negative electrode lead 225 are covered by the exterior body 207(see FIG. 8C).

Next, thermocompression bonding is performed along one side of theexterior body 207 that overlaps the positive electrode lead 221 and thenegative electrode lead 225. A portion where thermocompression bondingis performed along one side of the exterior body 207 is shown as abonding portion 207 a in FIG. 9A as in FIG. 8B.

Then, an electrolyte solution 204 is injected from an unsealed side 207b of the exterior body 207 illustrated in FIG. 9A. Then, the unsealedside 207 b of the exterior body 207 is sealed under vacuum, heat, andpressure. This treatment is performed in an environment from whichoxygen is eliminated, for example, in a glove box. The evacuation to avacuum may be performed with a vacuum sealer, a liquid pouring sealer,or the like. Heating and pressing can be performed by setting theexterior body 207 between two heatable bars included in the sealer. Anexample of the conditions is as follows: the degree of vacuum is 60 kPa,the heating temperature is 190 ° C., the pressure is 0.1 MPa, and thetime is 3 seconds. Here, pressure may be applied through the exteriorbody 207. The application of pressure enables removal of bubbles whichenter between the positive electrode and the negative electrode when theelectrolyte solution is injected.

Through the above-described process, the secondary battery 200 can befabricated (see FIG. 9B).

[6. After Aging Treatment, Performing Degasification and Resealing]

After the secondary battery 200 is fabricated through the above step,charging and discharging is preferably performed in the aging treatment.Furthermore, it is more preferable that gas caused by decomposition ofan electrolyte solution or the like at the time of aging treatment belet out and sealing be performed again.

The charging and discharging of the aging treatment is preferablyperformed while the secondary battery 200 is pressurized in thethickness direction (i.e., the direction parallel to the shortest of thelength, width, and height of the exterior body of the secondarybattery). By pressurizing the secondary battery 200, deformation such aswrinkles in the negative electrode can be further suppressed. Thepressing force can be 10 MPa, for example.

[7. Modification Example]

FIG. 10A shows the secondary battery 200 as a modification example ofthe secondary battery 200. The secondary battery 200 shown in FIG. 10Ais different from the secondary battery 200 shown in FIG. 4 in thearrangement of the positive electrode lead 221 and the negativeelectrode lead 225. Specifically, the positive electrode lead 221 andthe negative electrode lead 225 in the secondary battery 200 in FIG. 4are provided on the same side of the exterior body 207, whereas thepositive electrode lead 221 and the negative electrode lead 225 in thesecondary battery 200 in FIGS. 10A and 10B are provided on differentsides of the exterior body 207. The lead electrodes of the secondarybattery of one embodiment of the present invention can be freelypositioned as described above; therefore, the degree of freedom indesign is high. Accordingly, a product including the secondary batteryof one embodiment of the present invention can have a high degree offreedom in design. Furthermore, a yield of products each including thesecondary battery of one embodiment of the present invention can beincreased.

FIG. 10B illustrates a fabrication process of the secondary battery 200in FIG. 10A. The manufacturing method of the secondary battery 200 inFIG. 4 can be referred to for the details.

Pressing (e.g., embossing) may be performed to form projections anddepressions in advance on a surface of a film used as the exterior body207. The projections and depressions on the surface of the film increaseflexibility of a secondary battery and further relieves stress. Thedepressions or projections of a surface (or a rear surface) of the filmformed by embossing form an obstructed space that is sealed by the filmserving as a part of a wall of the sealing structure and whose innervolume is variable. It can be said that the depressions or projectionsof the film form an accordion structure (bellows structure) in thisobstructed space. Note that embossing, which is a kind of pressing, isnot necessarily employed and any method that allows formation of arelief on part of the film is employed.

Since a high capacitance material is used for the negative electrodeactive material, the negative electrode 111 of the secondary battery 200of one embodiment of the present invention can be made thin. Therefore,this structure is more preferable in the case where the secondarybattery 200 is curved or in the case where the secondary battery 200 hasflexibility.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and example.

EMBODIMENT 4

In this embodiment, other examples of the secondary battery of thepresent invention including the negative electrode described inEmbodiment 1 are described with reference to FIGS. 11A and 11B, FIGS.12A to 12C, FIGS. 13A to 13C, FIGS. 14A and 14B, FIGS. 15A1 to 15B2, andFIGS. 16A and 16B.

<Cylindrical Secondary Battery>

First, as another example of the secondary battery, a cylindricalsecondary battery is shown. A cylindrical secondary battery is describedwith reference to FIGS. 11A and 11B. As illustrated in FIG. 11A, acylindrical secondary battery 600 includes a positive electrode cap(battery cap) 601 on its top surface and a battery can (outer can) 602on its side surface and bottom surface. The positive electrode cap 601and the battery can 602 are insulated from each other by a gasket(insulating gasket) 610.

FIG. 11B is a schematic view of a cross-section of the cylindricalsecondary battery. Inside the battery can 602 having a hollowcylindrical shape, a battery element in which a strip-like positiveelectrode 604 and a strip-like negative electrode 606 are wound with aseparator 605 interposed therebetween is provided. Although notillustrated, the battery element is wound around a center pin as acenter. One end of the battery can 602 is close and the other endthereof is open. Alternatively, the battery can 602 is preferablycovered with nickel, aluminum, or the like in order to prevent corrosioncaused by a nonaqueous electrolytic solution. Alternatively, it ispreferable to cover the battery can 602 with nickel, aluminum, or thelike in order to prevent corrosion due to the electrolytic solution.Inside the battery can 602, the battery element in which the positiveelectrode, the negative electrode, and the separator are wound isinterposed between a pair of insulating plates 608 and 609 which faceeach other. Further, a nonaqueous electrolytic solution (notillustrated) is injected inside the battery can 602 provided with thebattery element. As the nonaqueous electrolytic solution, a nonaqueouselectrolytic solution which is similar to that of a secondary battery ofthe above embodiments can be used.

The positive electrode 604 and the negative electrode 606 can be formedin a manner similar to that of the positive electrode and the negativeelectrode of the thin secondary battery described in the aboveembodiment. Since the positive electrode and the negative electrode ofthe cylindrical secondary battery are wound, active materials are formedon both sides of the current collectors. A positive electrode terminal(positive electrode current collecting lead) 603 is connected to thepositive electrode 604, and a negative electrode terminal (negativeelectrode current collecting lead) 607 is connected to the negativeelectrode 606. Both the positive electrode terminal 603 and the negativeelectrode terminal 607 can be formed using a metal material such asaluminum. The positive electrode terminal 603 is welded to a safetyvalve mechanism 612, and the negative electrode teiininal 607 is weldedto the inner bottom of the battery can 602. The safety valve mechanism612 is electrically connected to the positive electrode cap 601 througha positive temperature coefficient (PTC) element 611. The safety valvemechanism 612 cuts off electrical connection between the positiveelectrode cap 601 and the positive electrode 604 when the internalpressure of the battery exceeds a predetermined threshold value.Further, the PTC element 611, which serves as a thermally sensitiveresistor whose resistance increases as temperature rises, limits theamount of current by increasing the resistance, in order to preventabnormal heat generation. Note that barium titanate (BaTiO₃)-basedsemiconductor ceramic or the like can be used for the PTC element.

Note that in FIGS. 11A and 11B, the cylindrical secondary battery isgiven as an example of the secondary battery; however, any of secondarybatteries with a variety of shapes, such as a sealed secondary batteryand a rectangular secondary battery, can be used. Further, a structurein which a plurality of positive electrodes, a plurality of negativeelectrodes, and a plurality of separators are stacked or wound may beemployed. For example, FIGS. 12A to 12C, FIGS. 13A to 13C, FIGS. 14A and14B, FIGS. 15A1 to 15B2, and FIGS. 16A and 16B illustrate examples ofother secondary batteries.

<Structure Example of Secondary Battery>

FIGS. 12A to 12C and FIGS. 13A to 13C illustrate structural examples ofthin secondary batteries. A wound body 993 illustrated in FIG. 12Aincludes a negative electrode 994, a positive electrode 995, and aseparator 996.

The wound body 993 is obtained by winding a sheet of a stack in whichthe negative electrode 994 overlaps with the positive electrode 995 withthe separator 996 provided therebetween. The wound body 993 is coveredwith a rectangular sealed container or the like; thus, a rectangularsecondary battery is fabricated.

Note that the number of stacks each including the negative electrode994, the positive electrode 995, and the separator 996 may be deteiminedas appropriate depending on capacity and an element volume which arerequired. The negative electrode 994 is connected to a negativeelectrode current collector (not illustrated) via one of a leadelectrode 997 and a lead electrode 998. The positive electrode 995 isconnected to a positive electrode current collector (not illustrated)via the other of the lead electrode 997 and the lead electrode 998.

In a secondary battery 980 illustrated in FIGS. 12B and 12C, the woundbody 993 is packed in a space formed by bonding a film 981 and a film982 having a depressed portion that serve as exterior bodies bythermocompression bonding or the like. The wound body 993 includes thelead electrode 997 and the lead electrode 998, and is soaked in anelectrolytic solution inside a space surrounded by the film 981 and thefilm 982 having a depressed portion.

For the film 981 and the film 982 having a depressed portion, a metalmaterial such as aluminum or a resin material can be used, for example.With the use of a resin material for the film 981 and the film 982having a depressed portion, the film 981 and the film 982 having adepressed portion can be deformed when external force is applied; thus,a flexible secondary battery can be manufactured.

Although FIGS. 12B and 12C illustrate an example where a space is formedby two films, the wound body 993 may be placed in a space formed bybending one film.

Furthermore, a flexible secondary battery can be fabricated when a resinmaterial or the like is used for the exterior body and the sealedcontainer of the thin secondary battery. Note that in the case where aresin material is used for the exterior body and the sealed container, aconductive material is used for a portion connected to the outside.

For example, FIGS. 13B and 13C illustrate another example of a flexiblethin secondary battery. The wound body 993 illustrated in FIG. 13A isthe same as that illustrated in FIG. 12A, and a detailed descriptionthereof is omitted.

In the secondary battery 990 illustrated in FIGS. 13B and 13C, the woundbody 993 is surrounded by the exterior body 991. The wound body 993includes the lead electrode 997 and the lead electrode 998, and issoaked in an electrolytic solution inside a space surrounded by theexterior body 991 and an exterior body 992. For example, a metalmaterial such as aluminum or a resin material can be used for theexterior bodies 991 and 992. With the use of a resin material for theexterior bodies 991 and 992, the exterior bodies 991 and 992 can bechanged in their forms when external force is applied; thus, a flexiblethin secondary battery can be fabricated.

<Structural Example of Power Storage System>

Structural examples of power storage systems will be described withreference to FIGS. 14A and 14B, FIGS. 15A1 to 15B2, and FIGS. 16A and16B. Here, a power storage system refers to, for example, a deviceincluding a secondary battery.

FIGS. 14A and 14B are external views of a power storage system. Thepower storage system includes a circuit board 900 and a secondarybattery 913. A label 910 is attached to the secondary battery 913. Asshown in FIG. 14B, the power storage system further includes a terminal951, a terminal 952, an antenna 914, and an antenna 915.

The circuit board 900 includes terminals 911 and a circuit 912. Theterminals 911 are connected to the terminals 951 and 952, the antennas914 and 915, and the circuit 912. Note that a plurality of terminals 911serving as a control signal input terminal, a power supply terminal, andthe like may be provided.

The circuit 912 may be provided on the rear side of the circuit board900. Each of the antennas 914 and 915 is not limited to having a coilshape and may have a linear shape or a plate shape. Further, a planarantenna, an aperture antenna, a traveling-wave antenna, an EH antenna, amagnetic-field antenna, or a dielectric antenna may be used.Alternatively, the antenna 914 or the antenna 915 may be a flat-plateconductor. The flat-plate conductor can serve as one of conductors forelectric field coupling. That is, the antenna 914 or the antenna 915 canserve as one of two conductors of a capacitor. Thus, power can betransmitted and received not only by an electromagnetic field or amagnetic field but also by an electric field.

The line width of the antenna 914 is preferably larger than that of theantenna 915. This makes it possible to increase the amount of electricpower received by the antenna 914.

The power storage system includes a layer 916 between the secondarybattery 913 and the antennas 914 and 915. The layer 916 has a functionof blocking an electromagnetic field from the secondary battery 913, forexample. As the layer 916, for example, a magnetic body can be used.

Note that the structure of the power storage system is not limited tothat shown in FIGS. 14A and 14B.

For example, as shown in FIGS. 15A1 and 15A2, two opposite surfaces ofthe secondary battery 913 in FIGS. 14A and 14B may be provided withrespective antennas. FIG. 15A1 is an external view showing one of theopposite sides, and FIG. 15A2 is an external view showing the other ofthe opposite sides. For portions similar to those in FIGS. 14A and 14B,a description of the power storage system illustrated in FIGS. 14A and14B can be referred to as appropriate.

As illustrated in FIG. 15A1, the antenna 914 is provided on one of theopposite surfaces of the secondary battery 913 with the layer 916interposed therebetween, and as illustrated in FIG. 15A2, the antenna915 is provided on the other of the opposite surfaces of the secondarybattery 913 with a layer 917 interposed therebetween. The layer 917 hasa function of blocking an electromagnetic field from the secondarybattery 913. As the layer 917, for example, a magnetic body can be used.

With the above structure, both of the antennas 914 and 915 can beincreased in size.

Alternatively, as illustrated in FIGS. 15B1 and 15B2, two oppositesurfaces of the secondary battery 913 in FIGS. 14A and 14B may beprovided with different types of antennas. FIG. 15B1 is an external viewshowing one of the opposite sides, and FIG. 15B2 is an external viewshowing the other of the opposite sides. For portions similar to thosein FIGS. 14A and 14B, a description of the power storage systemillustrated in FIGS. 14A and 14B can be referred to as appropriate.

As illustrated in FIG. 15B1, the antenna 914 and the antenna 915 areprovided on one of the opposing surfaces of the secondary battery 913with the layer 916 provided therebetween, and as illustrated in FIG.15B2, an antenna 918 is provided on the other of the opposing surfacesof the secondary battery 913 with the layer 917 provided therebetween.The antenna 918 has a function of performing data communication with anexternal device, for example. An antenna with a shape that can beapplied to the antennas 914 and 915, for example, can be used as theantenna 918. As an example of a method for communication between thepower storage system and another device via the antenna 918, a responsemethod that can be used between the power storage system and anotherdevice, such as NFC, can be employed.

Alternatively, as illustrated in FIG. 16A, the secondary battery 913 inFIGS. 14A and 14B may be provided with a display device 920. The displaydevice 920 is electrically connected to the terminal 911 via a terminal919. It is possible that the label 910 is not provided in a portionwhere the display device 920 is provided. For portions similar to thosein FIGS. 14A and 14B, a description of the power storage systemillustrated in FIGS. 14A and 14B can be referred to as appropriate.

The display device 920 can display, for example, an image showingwhether or not charging is being carried out, an image showing theamount of stored power, or the like. As the display device 920,electronic paper, a liquid crystal display device, an electroluminescent(EL) display device, or the like can be used. For example, powerconsumption of the display device 920 can be reduced when electronicpaper is used.

Alternatively, as illustrated in FIG. 16B, the secondary battery 913illustrated in FIGS. 14A and 14B may be provided with a sensor 921. Thesensor 921 is electrically connected to the terminal 911 via a terminal922. For portions similar to those in FIGS. 14A and 14B, a descriptionof the power storage system illustrated in FIGS. 14A and 14B can bereferred to as appropriate.

As the sensor 921, a sensor that has a function of measuring, forexample, force, displacement, position, speed, acceleration, angularvelocity, rotational frequency, distance, light, liquid, magnetism,temperature, chemical substance, sound, time, hardness, electric field,electric current, voltage, electric power, radiation, flow rate,humidity, gradient, oscillation, odor, or infrared rays can be used.With the sensor 921, for example, data on the environment (e.g.,temperature) where the power storage system is placed can be acquiredand stored in a memory in the circuit 912.

The electrode of one embodiment of the present invention is used in thesecondary battery and the power storage system that are described inthis embodiment. The capacity of the secondary battery and the powerstorage system can thus be high. Furthermore, energy density can behigh. Moreover, reliability can be high, and life can be long.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

EMBODIMENT 5

A battery management unit (BMU) which can be combined with the secondarybattery including the negative electrode described in the aboveembodiment and a transistor suitable for a circuit included in thebattery management unit are described with reference to FIG. 17, FIGS.18A to 18C, FIG. 19, FIG. 20, FIGS. 21A to 21C, FIG. 22, and. FIG. 23.In this embodiment, a battery management unit of a power storage deviceincluding battery cells that are connected in series is particularlydescribed.

When a plurality of battery cells connected in series are charged anddischarged repeatedly, each battery cell has different capacity (outputvoltage) from one another due to the variation in charge and dischargecharacteristics among the battery cells. A discharge capacity of all ofthe plurality of battery cells connected in series depends on a batterycell with small capacity. The variation in capacities among the batterycells reduces the capacity of the all the battery cells at the time ofdischarging. Charging based on a battery cell with small capacity maycause insufficient charging. Charging based on a battery cell with highcapacity may cause overcharge.

Thus, the battery management unit of the power storage device includingbattery cells connected in series has a function of reducing variationin capacities among the battery cells which causes insufficient chargingor overcharge. Although circuit structures for reducing variation incapacities among the battery cells include a resistive type, a capacitortype, and an inductor type, here, a circuit structure which can reducevariation in capacities among the battery cells using transistors with alow off-state current is explained as an example.

A transistor including an oxide semiconductor in its channel formationregion (an OS transistor) is preferably used as the transistor with alow off-state current. When an OS transistor with a low off-statecurrent is used in the circuit of the battery management unit of thepower storage device, the amount of electric charge leaking from abattery can be reduced, and reduction in capacity with the lapse of timecan be suppressed.

As the oxide semiconductor used in the channel formation region, anIn-M-Zn oxide (M is Ga, Sn, Y, Zr, La, Cc, or Nd) is used. In the casewhere the atomic ratio of the metal elements of a target for forming anoxide semiconductor film is In:M:Zn=x₁:y₁:z₁, x₁/y₁ is preferablygreater than or equal to ⅓ and less than or equal to 6, furtherpreferably greater than or equal to 1 and less than or equal to 6, andz₁/y₁ is preferably greater than or equal to ⅓ and less than or equal to6, further preferably greater than or equal to 1 and less than or equalto 6. Note that when z₁/y₁ is greater than or equal to 1 and less thanor equal to 6, a CAAC-OS film as the oxide semiconductor film is easilyformed.

Here, the details of the CAAC-OS film are described.

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OSfilm, which is obtained using a transmission electron microscope (TEM),a plurality of crystal parts can be observed. However, in thehigh-resolution TEM image, a boundary between crystal parts, that is, agrain boundary is not clearly observed. Thus, in the CAAC-OS film, areduction in electron mobility due to the grain boundary is less likelyto occur.

According to the high-resolution cross-sectional TEM image of theCAAC-OS film observed in a direction substantially parallel to a samplesurface, metal atoms are arranged in a layered manner in the crystalparts. Each metal atom layer has a morphology reflecting unevenness of asurface over which the CAAC-OS film is formed (hereinafter, a surfaceover which the CAAC-OS film is formed is referred to as a formationsurface) or of a top surface of the CAAC-OS film, and is arrangedparallel to the formation surface or the top surface of the CAAC-OSfilm.

On the other hand, according to the high-resolution planar TEM image ofthe CAAC-OS film observed in a direction substantially perpendicular tothe sample surface, metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ not 36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, a transistorincluding the oxide semiconductor film rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has fewcarrier traps. Accordingly, the transistor including the oxidesemiconductor film has little variation in electrical characteristicsand high reliability. Electric charge trapped by the carrier traps inthe oxide semiconductor film takes a long time to be released, and mightbehave like fixed electric charge. Thus, the transistor which includesthe oxide semiconductor film having high impurity concentration and ahigh density of defect states has unstable electrical characteristics insome cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

Since the OS transistor has a wider band gap than a transistor includingsilicon in its channel formation region (a Si transistor), dielectricbreakdown is unlikely to occur when a high voltage is applied. Althougha voltage of several hundreds of volts is generated when battery cellsare connected in series, the above described OS transistor is suitablefor a circuit of a battery management unit which is used for suchbattery cells in the power storage device.

FIG. 17 is an example of a block diagram of the power storage device. Apower storage device BT00 illustrated in FIG. 17 includes a terminalpair BT01, a terminal pair BT02, a switching control circuit BT03, aswitching circuit BT04, a switching circuit BT05, a voltagetransformation control circuit BT06, a voltage transformer circuit BT07,and a battery portion BT08 including a plurality of battery cells BT09connected in series.

In the power storage device BT00 illustrated in FIG. 17, a portionincluding the terminal pair BT01, the terminal pair BT02, the switchingcontrol circuit BT03, the switching circuit BT04, the switching circuitBT05, the voltage transformation control circuit BT06, and the voltagetransformer circuit BT07 can be referred to as a battery managementunit.

The switching control circuit BT03 controls operations of the switchingcircuits BT04 and BT05. Specifically, the switching control circuit BT03selects battery cells to be discharged (a discharge battery cell group)and battery cells to be charged (a charge battery cell group) inaccordance with voltage measured for every battery cell BT09.

Furthermore, the switching control circuit BT03 outputs a control signalS1 and a control signal S2 on the basis of the selected dischargebattery cell group and the selected charge battery cell group. Thecontrol signal S1 is output to the switching circuit BT04. The controlsignal S1 controls the switching circuit BT04 so that the terminal pairBT01 and the discharge battery cell group are connected. In addition,the control signal S2 is output to the switching circuit BT05. Thecontrol signal S2 controls the switching circuit BT05 so that theterminal pair BT02 and the charge battery cell group are connected.

The switching control circuit BT03 generates the control signal S1 andthe control signal S2 on the basis of connection relation of theswitching circuit BT04, the switching circuit BT05, and the voltagetransformer circuit BT07 so that terminals having the same polarity ofthe terminal pair BT01 and the discharge battery cell group areconnected with each other, or terminals having the same polarity of theterminal pair BT02 and the charge battery cell group are connected witheach other.

An operation of the switching control circuit BT03 is described indetail.

First, the switching control circuit BT03 measures the voltage of eachof the plurality of battery cells BT09. Then, the switching controlcircuit BT03 deteiinines that the battery cell BT09 having a voltagehigher than a predetermined threshold value is a high-voltage batterycell (high-voltage cell) and that a battery cell BT09 having a voltagelower than the predetermined threshold value is a low-voltage batterycell (low-voltage cell), for example.

As a method to determine whether a battery cell is a high-voltage cellor a low-voltage cell, any of various methods can be employed. Forexample, the switching control circuit BT03 may determine whether eachbattery cell BT09 is a high-voltage cell or a low-voltage cell on thebasis of the voltage of a battery cell BT09 having the highest voltageor the lowest voltage among the plurality of battery cells BT09. In thiscase, the switching control circuit BT03 can determine whether eachbattery cell BT09 is a high-voltage cell or a low-voltage cell bydetermining whether or not a ratio of a voltage of each battery cellBT09 to the reference voltage is the predetermined value or more. Then,the switching control circuit BT03 determines a charge battery cellgroup and a discharge battery cell group on the basis of thedetermination result.

Note that high-voltage cells and low-voltage cells are mixed in variousstates in the plurality of battery cells BT09. For example, theswitching control circuit BT03 selects a portion having the largestnumber of high-voltage cells connected in series as the dischargebattery cell group of mixed high-voltage cells and low-voltage cells.Furthermore, the switching control circuit BT03 selects a portion havingthe largest number of low-voltage cells connected in series as thecharge battery cell group. In addition, the switching control circuitBT03 may preferentially select battery cells BT09 which are nearovercharge or overdischarge as the discharge battery cell group or thecharge battery cell group.

Here, operation examples of the switching control circuit BT03 in thisembodiment are described with reference to FIGS. 18A to 18C. FIGS. 18Ato 18C illustrate operation examples of the switching control circuitBT03. Note that FIGS. 18A to 18C each illustrate the case where fourbattery cells BT09 are connected in series as an example for convenienceof explanation.

FIG. 18A shows the case where the relation of voltages Va, Vb, Vc, andVd is Va=Vb=Vc>Vd where the voltages Va, Vb, Vc, and Vd are voltages ofa battery cell a, a battery cell b, a battery cell c, and a battery celld, respectively. That is, a series of three high-voltage cells a to cand one low-voltage cell d are connected in series. In that case, theswitching control circuit BT03 selects the series of three high-voltagecells a to c as the discharge battery cell group. In addition, theswitching control circuit BT03 selects the low-voltage cell d as thecharge battery cell group.

Next, FIG. 18B shows the case where the relation of the voltages isVc>Va=Vb>>Vd. That is, a series of two low-voltage cells a and b, onehigh-voltage cell c, and one low-voltage cell d which is close tooverdischarge are connected in series. In that case, the switchingcontrol circuit BT03 selects the high-voltage cell c as the dischargebattery cell group. Since the low-voltage cell d is close tooverdischarge, the switching control circuit BT03 preferentially selectsthe low-voltage cell d as the charge battery cell group instead of theseries of two low-voltage cells a and b.

Lastly, FIG. 18C shows the case where the relation of the voltages isVa>Vb=Vc=Vd. That is, one high-voltage cell a and a series of threelow-voltage cells b to d are connected in series. In that case, theswitching control circuit BT03 selects the high-voltage cell a as thedischarge battery cell group. In addition, the switching control circuitBT03 selects the series of three low-voltage cells b to d as the chargebattery cell group.

On the basis of the determination result shown in the examples of FIGS.18A to 18C, the switching control circuit BT03 outputs the controlsignal SI and the control signal S2 to the switching circuit BT04 andthe switching circuit BT05, respectively. Information showing thedischarge battery cell group being the connection destination of theswitching circuit BT04 is set in the control signal S1. Informationshowing the charge battery cell group being a connection destination ofthe switching circuit BT05 is set in the control signal S2.

The above is the detailed description of the operation of the switchingcontrol circuit BT03.

The switching circuit BT04 sets the discharge battery cell groupselected by the switching control circuit BT03 as the connectiondestination of the terminal pair BT01 in response to the control signalSi output from the switching control circuit BT03.

The terminal pair BT01 includes a pair of terminals A1 and A2. Theswitching circuit BT04 sets the connection destination of the terminalpair BT01 by connecting one of the pair of terminals A1 and A2 to apositive electrode terminal of a battery cell BT09 positioned on themost upstream side (on the high potential side) of the discharge batterycell group, and the other to a negative electrode terminal of a batterycell BT09 positioned on the most downstream side (on the low potentialside) of the discharge battery cell group. Note that the switchingcircuit BT04 can recognize the position of the discharge battery cellgroup on the basis of the information set in the control signal S1.

The switching circuit BT05 sets the charge battery cell group selectedby the switching control circuit BT03 as the connection destination ofthe terminal pair BT02 in response to the control signal S2 output fromthe switching control circuit BT03.

The terminal pair BT02 includes a pair of terminals B1 and B2. Theswitching circuit BT05 sets the connection destination of the terminalpair BT02 by connecting one of the pair of terminals B1 and B2 to apositive electrode terminal of a battery cell BT09 positioned on themost upstream side (on the high potential side) of the charge batterycell group, and the other to a negative electrode terminal of a batterycell BT09 positioned on the most downstream side (on the low potentialside) of the charge battery cell group. Note that the switching circuitBT05 can recognize the position of the charge battery cell group on thebasis of the information set in the control signal S2.

FIG. 19 and FIG. 20 are circuit diagrams showing configuration exampleof the switching circuits BT04 and BT05.

In FIG. 19, the switching circuit BT04 includes a plurality of thetransistors BT10, a bus BT11, and a bus BT12. The bus BT11 is connectedto the terminal A1. The bus BT12 is connected to the terminal A2.Sources or drains of the plurality of transistors BT10 are connectedalternately to the bus BT11 and the bus BT12. Sources or drains whichare not connected to the bus BT11 and the bus BT12 of the plurality ofthe transistors BT10 are each connected between two adjacent batterycells BT09.

A source or a drain of the transistor BT10 which is not connected to thebus BT11 on the most upstream side of the plurality of transistors BT10is connected to a positive electrode terminal of a battery cell BT09 onthe most upstream side of the battery portion BT08. A source or a drainof a transistor BT10 which is not connected to the bus BT11 of thetransistor BT10 on the most downstream side of the plurality oftransistors BT10 is connected to a negative electrode terminal of abattery cell BT09 on the most downstream side of the battery portionBT08.

The switching circuit BT04 connects the discharge battery cell group tothe terminal pair BT01 by bringing one of the plurality of transistorsBT10 which are connected to the bus BT11 and one of the plurality oftransistors BT10 which are connected to the bus BT12 into an on state inresponse to the control signal S1 supplied to gates of the plurality oftransistors BT10. Accordingly, the positive electrode terminal of thebattery cell BT09 on the most upstream side of the discharge batterycell group is connected to one of the pair of terminals A1 and A2. Inaddition, the negative electrode terminal of the battery cell BT09 onthe most downstream side of the discharge battery cell group isconnected to the other of the pair of terminals A1 and A2 (i.e., aterminal which is not connected to the positive electrode terminal).

OS transistors are preferably used as the transistors BT10. Since theoff-state current of the OS transistor is low, the amount of electriccharge leaking from battery cells which do not belong to the dischargebattery cell group can be reduced, and reduction in capacity with thelapse of time can be suppressed. In addition, dielectric breakdown isunlikely to occur in the OS transistor when a high voltage is applied.Therefore, the battery cell BT09 and the terminal pair BT01, which areconnected to the transistor BT10 in an off state, can be insulated fromeach other even when an output voltage of the discharge battery cellgroup is high.

In FIG. 19, the switching circuit BT05 includes a plurality oftransistors BT13, a current control switch BT14, a bus BT15, and a busBT16. The bus BT15 and the bus BT16 are provided between the pluralityof transistors BT13 and the current control switch BT14. Sources ordrains of the plurality of transistors BT13 are connected alternately tothe bus BT15 and the bus BT16. Sources or drains which are not connectedto the bus BT15 and the bus BT16 of the plurality of transistors BT13are each connected between two adjacent battery cells BT09.

A source or a drain of a transistor BT13 which is not connected to thebus BT15 on the most upstream side of the plurality of transistors BT13is connected to the positive electrode terminal of the battery cell BT09on the most upstream side of the battery portion BT08. A source or adrain of a transistor BT13 which is not connected to the bus BT15 on themost downstream side of the plurality of transistors BT13 is connectedto the negative electrode terminal of the battery cell BT09 on the mostdownstream side of the battery portion BT08.

OS transistors are preferably used as the transistors BT13 like thetransistors BT10. Since the off-state current of the OS transistor islow, the amount of electric charge leaking from the battery cells whichdo not belong to the charge battery cell group can be reduced, andreduction in capacity with the lapse of time can be suppressed. Inaddition, dielectric breakdown is unlikely to occur in the OS transistorwhen a high voltage is applied. Therefore, the battery cell BT09 and theterminal pair BT02, which are connected to the transistor BT13 in an offstate, can be insulated from each other even when a voltage for chargingthe charge battery cell group is high.

The current control switch BT14 includes a switch pair BT17 and a switchpair BT18. One end of the switch pair BT17 is connected to the terminalB1. The other ends of the switch pair BT17 extend from respectiveswitches. One switch is connected to the bus BT15, and the other switchis connected to the bus BT16. One end of the switch pair RT18 isconnected to the terminal B2. The other ends of the switch pair BT18extend from respective switches. One switch is connected to the busBT15, and the other switch is connected to the bus BT16.

OS transistors are preferably used for the switches included in theswitch pair BT17 and the switch pair BT18 like the transistors BT10 andBT13.

The switching circuit BT05 connects the charge battery cell group andthe terminal pair BT02 by controlling the combination of on and offstates of the transistors BT13 and the current control switch BT14 inresponse to the control signal S2.

For example, the switching circuit BT05 connects the charge battery cellgroup and the terminal pair BT02 in the following manner.

The switching circuit BT05 brings a transistor BT13 connected to thepositive electrode terminal of a battery cell BT09 on the most upstreamside of the charge battery cell group into an on state in response tothe control signal S2 supplied to gates of the plurality of thetransistors BT13. In addition, the switching circuit BT05 brings atransistor BT13 connected to the negative electrode terminal of abattery cell BT09 on the most downstream side of the charge battery cellgroup into an on state in response to the control signal S2 supplied tothe gates of the plurality of the transistors BT13.

The polarities of voltages applied to the terminal pair BT02 can vary inaccordance with the connection structures of the voltage transformercircuit BT07 and the discharge battery cell group connected to theterminal pair BT01. In order to supply current in a direction forcharging the charge battery cell group, terminals with the same polarityof the terminal pair BT02 and the charge battery cell group are requiredto be connected. In view of this, the current control switch BT14 iscontrolled by the control signal S2 so that the connection destinationof the switch pair BT17 and that of the switch pair BT18 are changed inaccordance with the polarities of the voltages applied to the terminalpair BT02.

The state where voltages are applied to the terminal pair BT02 so as tomake the terminal B1 a positive electrode and the terminal B2 a negativeelectrode is described as an example. Here, in the case where thebattery cell BT09 positioned on the most downstream side of the batteryportion BT08 is in the charge battery cell group, the switch pair BT17is controlled to be connected to the positive electrode terminal of thebattery cell BT09 in response to the control signal S2. That is, theswitch of the switch pair BT17 connected to the bus BT16 is turned on,and the switch of the switch pair BT17 connected to the bus BT15 isturned off. In contrast, the switch pair BT18 is controlled to beconnected to the negative electrode terminal of the battery cell BT09positioned on the most downstream side of the battery portion BT08 inresponse to the control signal S2. That is, the switch of the switchpair BT18 connected to the bus BT15 is turned on, and the switch of theswitch pair BT18 connected to the bus BT16 is turned off. In thismanner, terminals with the same polarity of the terminal pair BT02 andthe charge battery cell group are connected to each other. In addition,the current which flows from the terminal pair BT02 is controlled to besupplied in a direction so as to charge the charge battery cell group.

In addition, instead of the switching circuit BT05, the switchingcircuit BT04 may include the current control switch BT14. In that case,the polarities of the voltages applied to the terminal pair BT02 arecontrolled by controlling the polarities of the voltages applied to theterminal pair BT01 in response to the operation of the current controlswitch BT14 and the control signal S1. Thus, the current control switchBT14 controls the direction of current which flows to the charge batterycell group from the terminal pair BT02.

FIG. 20 is a circuit diagram illustrating structure examples of theswitching circuit BT04 and the switching circuit BT05 which aredifferent from those of FIG. 19.

In FIG. 20, the switching circuit BT04 includes a plurality oftransistor pairs BT21, a bus BT24, and a bus BT25. The bus BT24 isconnected to the terminal A1.

The bus BT25 is connected to the terminal A2. One ends of the pluralityof transistor pairs BT21 extend from transistors BT22 and transistorsBT23. Sources or drains of the transistors BT22 are connected to the busBT24. Sources or drains of the transistors BT23 are connected to the busBT25. In addition, the other ends of the plurality of transistor pairsare each connected between two adjacent battery cells BT09. The otherend of the transistor pair BT21 on the most upstream side of theplurality of transistor pairs BT21 is connected to the positiveelectrode terminal of the battery cell BT09 on the most upstream side ofthe battery portion BT08. The other end of the transistor pair BT21 onthe most downstream side of the plurality of transistor pairs BT21 isconnected to a negative electrode terminal of the battery cell BT09 onthe most downstream side of the battery portion BT08.

The switching circuit BT04 switches the connection destination of thetransistor pair BT21 to one of the terminal A1 and the terminal A2 byturning on or off the transistors BT22 and BT23 in response to thecontrol signal S1. Specifically, when the transistor BT22 is turned on,the transistor BT23 is turned off, so that the connection destination ofthe transistor pair BT21 is the terminal A1. On the other hand, when thetransistor BT23 is turned on, the transistor BT22 is turned off, so thatthe connection destination of the transistor pair BT21 is the terminalA2. Which of the transistors BT22 and BT23 is turned on is determined bythe control signal S1.

Two transistor pairs BT21 are used to connect the terminal pair BT01 andthe discharge battery cell group. Specifically, the connectiondestinations of the two transistor pairs BT21 are determined on thebasis of the control signal S1, and the discharge battery cell group andthe terminal pair BT01 are connected. The connection destinations of thetwo transistor pairs BT21 are controlled by the control signal S1 sothat one of the connection destinations is the terminal A1 and the otheris the terminal A2.

The switching circuit BT05 includes a plurality of transistor pairsBT31, a bus BT34 and a bus BT35. The bus BT34 is connected to theterminal B1. The bus BT35 is connected to the terminal B2. One ends ofthe plurality of transistor pairs BT31 extend from transistors BT32 andtransistors BT33. One end extending from the transistor BT32 isconnected to the bus BT34. One end extending from the transistor BT33 isconnected to the bus BT35. The other ends of the plurality of transistorpairs BT31 are each connected between two adjacent battery cells BT09.The other end of the transistor pair BT31 on the most upstream side ofthe plurality of transistor pairs BT31 is connected to the positiveelectrode terminal of the battery cell BT09 on the most upstream side ofthe battery portion BT08. The other end of the transistor pair BT31 onthe most downstream side of the plurality of transistor pairs BT31 isconnected to the negative electrode terminal of the battery cell BT09 onthe most downstream side of the battery portion BT08.

The switching circuit BT05 switches the connection destination of thetransistor pair BT31 to one of the terminal B1 and the terminal B2 byturning on or off the transistors BT32 and BT33 in response to thecontrol signal S2. Specifically, when the transistor BT32 is turned on,the transistor BT33 is turned off, so that the connection destination ofthe transistor pair BT31 is the terminal B1. On the other hand, when thetransistor BT33 is turned on, the transistor BT32 is turned off, so thatthe connection destination of the transistor pair BT31 is the terminalB2. Which of the transistors BT32 and BT33 is turned on is determined bythe control signal S2.

Two transistor pairs BT31 are used to connect the terminal pair BT02 andthe charge battery cell group. Specifically, the connection destinationsof the two transistor pairs BT31 are determined on the basis of thecontrol signal S2, and the charge battery cell group and the terminalpair BT02 are connected. The connection destinations of the twotransistor pairs BT31 are controlled by the control signal S2 so thatone of the connection destinations is the terminal B1 and the other isthe terminal B2.

The connection destinations of the two transistor pairs BT31 aredetermined by the polarities of the voltages applied to the terminalpair BT02. Specifically, in the case where voltages which make theterminal B1 a positive electrode and the terminal B2 a negativeelectrode are applied to the terminal pair BT02, the transistor pairBT31 on the upstream side is controlled by the control signal S2 so thatthe transistor BT32 is turned on and the transistor BT33 is turned off.In contrast, the transistor pair BT31 on the downstream side iscontrolled by the control signal S2 so that the transistor BT33 isturned on and the transistor BT32 is turned off. In the case wherevoltages which make the terminal B1 a negative electrode and theterminal B2 a positive electrode are applied to the terminal pair BT02,the transistor pair BT31 on the upstream side is controlled by thecontrol signal S2 so that the transistor BT33 is turned on and thetransistor BT32 is turned off. In contrast, the transistor pair BT31 onthe downstream side is controlled by the control signal S2 so that thetransistor BT32 is turned on and the transistor BT33 is turned off. Inthis manner, terminals with the same polarity of the terminal pair BT02and the charge battery cell group are connected to each other. Inaddition, the current which flows from the terminal pair BT02 iscontrolled to be supplied in a direction for charging the charge batterycell group.

The voltage transformation control circuit BT06 controls operation ofthe voltage transformer circuit BT07. The voltage transformation controlcircuit BT06 generates a voltage transformation signal S3 forcontrolling the operation of the voltage transformer circuit BT07 on thebasis of the number of the battery cells BT09 included in the dischargebattery cell group and the number of the battery cells BT09 included inthe charge battery cell group and outputs the voltage transfoiniationsignal S3 to the voltage transformer circuit BT07.

In the case where the number of the battery cells BT09 included in thedischarge battery cell group is larger than that included in the chargebattery cell group, it is necessary to prevent a charging voltage whichis too high from being applied to the charge battery cell group. Thus,the voltage transformation control circuit BT06 outputs the voltagetransformation signal S3 for controlling the voltage transformer circuitBT07 so that a discharging voltage (Vdis) is lowered within a rangewhere the charge battery cell group can be charged.

In the case where the number of the battery cells BT09 included in thedischarge battery cell group is less than or equal to that included inthe charge battery cell group, a voltage necessary for charging thecharge battery cell group needs to be secured. Therefore, the voltagetransformation control circuit BT06 outputs the voltage transformationsignal S3 for controlling the voltage transformer circuit BT07 so thatthe discharging voltage (Vdis) is raised within a range where a chargingvoltage which is too high is not applied to the charge battery cellgroup.

The voltage value of the charging voltage which is too high isdeteiiiiined in the light of product specifications and the like of thebattery cell BT09 used in the battery portion BT08. The voltage which israised or lowered by the voltage transformer circuit BT07 is applied asa charging voltage (Vcha) to the terminal pair BT02.

Here, operation examples of the voltage transformation control circuitBT06 in this embodiment are described with reference to FIGS. 21A to21C. FIGS. 21A to 21C are conceptual diagrams for explaining theoperation examples of the voltage transformation control circuit BT06corresponding to the discharge battery cell group and the charge batterycell group described in FIGS. 18A to 18C. FIGS. 21A to 21C eachillustrate a battery management unit BT41. The battery management unitBT41 includes the terminal pair BT01, the terminal pair BT02, theswitching control circuit BT03, the switching circuit BT04, theswitching circuit BT05, the voltage transformation control circuit BT06,and the voltage transformer circuit BT07.

In an example illustrated in FIG. 21A, the series of three high-voltagecells a to c and one low-voltage cell d are connected in series asdescribed in FIG. 18A. In that case, as described using FIG. 18A, theswitching control circuit BT03 selects the high-voltage cells a to c asthe discharge battery cell group, and selects the low-voltage cell d asthe charge battery cell group. The voltage transformation controlcircuit BT06 calculates a conversion ratio N for converting thedischarging voltage (Vdis) to the charging voltage (Vcha) based on theratio of the number of the battery cells BT09 included in the chargebattery cell group to the number of the battery cells BT09 included inthe discharge battery cell group.

In the case where the number of the battery cells BT09 included in thedischarge battery cell group is larger than that included in the chargebattery cell group, when a discharging voltage is applied to theterminal pair BT02 without transforming the voltage, overvoltage may beapplied to the battery cells BT09 included in the charge battery cellgroup through the terminal pair BT02. Thus, in the case of FIG. 21A, itis necessary that a charging voltage (Vcha) applied to the terminal pairBT02 be lower than the discharging voltage. In addition, in order tocharge the charge battery cell group, it is necessary that the chargingvoltage be higher than the total voltage of the battery cells BT09included in the charge battery cell group. Thus, the voltagetransformation control circuit BT06 sets the conversion ratio N largerthan the ratio of the number of the battery cells BT09 included in thecharge battery cell group to the number of the battery cells BT09included in the discharge battery cell group.

Thus, the voltage transformation control circuit BT06 preferably setsthe conversion ratio N larger than the ratio of the number of thebattery cells BT09 included in the charge battery cell group to thenumber of the battery cells BT09 included in the discharge battery cellgroup by about 1% to 10%. Here, the charging voltage is made higher thanthe voltage of the charge battery cell group, but the charging voltageis equal to the voltage of the charge battery cell group in reality.Note that the voltage transformation control circuit BT06 feeds acurrent for charging the charge battery cell group in accordance withthe conversion ratio N in order to make the voltage of the chargebattery cell group equal to the charging voltage. The value of thecurrent is set by the voltage transformation control circuit BT06.

In the example illustrated in FIG. 21A, since the number of the batterycells BT09 included in the discharge battery cell group is three and thenumber of the battery cells BT09 included in the charge battery cellgroup is one, the voltage transformation control circuit BT06 calculatesa value which is slightly larger than ⅓ as the conversion ratio N. Then,the voltage transformation control circuit BT06 outputs the voltagetransformation signal S3, which lowers the discharging voltage inaccordance with the conversion ratio N and converts the voltage into acharging voltage, to the voltage transformer circuit BT07. The voltagetransformer circuit BT07 applies the charging voltage which is obtainedby transformation in response to the voltage transformation signal S3 tothe terminal pair BT02. Then, the battery cells BT09 included in thecharge battery cell group are charged with the charging voltage appliedto the terminal pair BT02.

In each of examples illustrated in FIGS. 21B and 21C, the conversionratio Nis calculated in a manner similar to that of FIG. 21A. In each ofthe examples illustrated in FIGS. 21B and 21C, since the number of thebattery cells BT09 included in the discharge battery cell group is lessthan or equal to the number of the battery cells BT09 included in thecharge battery cell group, the conversion ratio N is 1 or more.Therefore, in this case, the voltage transformation control circuit BT06outputs the voltage transformation signal S3 for raising the dischargingvoltage and converting the voltage into the charging voltage.

The voltage transformer circuit BT07 converts the discharging voltageapplied to the terminal pair BT01 into a charging voltage on the basisof the voltage transformation signal S3. The voltage transformer circuitBT07 applies the converted charging voltage to the terminal pair BT02.Here, the voltage transformer circuit BT07 electrically insulates theterminal pair BT01 from the terminal pair BT02. Accordingly, the voltagetransformer circuit BT07 prevents a short circuit due to a differencebetween the absolute voltage of the negative electrode terminal of thebattery cell BT09 on the most downstream side of the discharge batterycell group and the absolute voltage of the negative electrode terminalof the battery cell BT09 on the most downstream side of the chargebattery cell group. Furthermore, the voltage transformer circuit BT07converts the discharging voltage, which is the total voltage of thedischarge battery cell group, into the charging voltage on the basis ofthe voltage transformation signal S3 as described above.

An insulated direct current (DC)-DC converter or the like can be used inthe voltage transformer circuit BT07. In that case, the voltagetransformation control circuit BT06 controls the charging voltageconverted by the voltage transformer circuit BT07 by outputting a signalfor controlling the on/off ratio (the duty ratio) of the insulated DC-DCconverter as the voltage transformation signal S3.

Examples of the insulated DC-DC converter include a flyback converter, aforward converter, a ringing choke converter (RCC), a push-pullconverter, a half-bridge converter, and a full-bridge converter, and asuitable converter is selected in accordance with the value of theintended output voltage.

The structure of the voltage transformer circuit BT07 including theinsulated DC-DC converter is illustrated in FIG. 22. An insulated DC-DCconverter BT51 includes a switch portion BT52 and a transformer BT53.The switch portion BT52 is a switch for switching on/off of theinsulated DC-DC converter, and a metal oxide semiconductor field-effecttransistor (MOSFET), a bipolar transistor, or the like is used as theswitch portion BT52. The switch portion BT52 periodically turns on andoff the insulated DC-DC converter BT51 in accordance with the voltagetransformation signal S3 controlling the on/off ratio which is outputfrom the voltage transformation control circuit BT06. The switch portionBT52 can have any of various structures in accordance with the type ofthe insulated DC-DC converter which is used. The transformer BT53converts the discharging voltage applied from the terminal pair BT01into the charging voltage. In detail, the transformer BT53 operates inconjunction with the on/off state of the switch portion BT52 andconverts the discharging voltage into the charging voltage in accordancewith the on/off ratio. As the time during which the switch portion BT52is on becomes longer in its switching period, the charging voltage isincreased. On the other hand, as the time during which the switchportion BT52 is on becomes shorter in its switching period, the chargingvoltage is decreased. In the case where the insulated DC-DC converter isused, the terminal pair BT01 and the terminal pair BT02 can be insulatedfrom each other inside the transformer BT53.

A flow of operation of the power storage device BT00 in this embodimentis described with reference to FIG. 23. FIG. 23 is a flow chartillustrating the flow of the operation of the power storage device BT00.

First, the power storage device BT00 obtains a voltage measured for eachof the plurality of battery cells BT09 (step S001). Then, the powerstorage device BT00 determines whether or not the condition for startingthe operation of suppressing variation in voltages of the plurality ofbattery cells BT09 lower than the predetermined threshold value issatisfied (step S002). An example of the condition can be that thedifference between the maximum value and the minimum value of thevoltage measured for each of the plurality of battery cells BT09 ishigher than or equal to the predetermined threshold value. In the casewhere the condition is not satisfied (step S002: NO), the power storagedevice BT00 does not perform the following operation because voltages ofthe battery cells BT09 are well balanced. In contrast, in the case wherethe condition is satisfied (step S002: YES), the power storage deviceBT00 performs the operation of suppressing variation in the voltages ofthe battery cells BT09 lower than the predetermined threshold value. Inthis operation, the power storage device BT00 determines whether eachbattery cell BT09 is a high-voltage cell or a low-voltage cell on thebasis of the measured voltage of each cell (step S003). Then, the powerstorage device BT00 determines a discharge battery cell group and acharge battery cell group on the basis of the determination result (stepS004). In addition, the power storage device BT00 generates the controlsignal S1 for setting the determined discharge battery cell group as theconnection destination of the terminal pair BT01, and the control signalS2 for setting the determined charge battery cell group as theconnection destination of the terminal pair BT02 (step S005). The powerstorage device BT00 outputs the generated control signals S1 and S2 tothe switching circuit BT04 and the switching circuit BT05, respectively.Then, the switching circuit BT04 connects the terminal pair BT01 and thedischarge battery cell group, and the switching circuit BT05 connectsthe terminal pair BT02 and the discharge battery cell group (step S006).The power storage device BT00 generates the voltage transformationsignal S3 based on the number of the battery cells BT09 included in thedischarge battery cell group and the number of the battery cells BT09included in the charge battery cell group (step S007). Then, the powerstorage device BT00 converts the discharging voltage applied to theterminal pair BT01 into a charging voltage based on the voltagetransformation signal S3 and applies the charging voltage to theterminal pair BT02 (step S008). In this way, electric charge of thedischarge battery cell group is transferred to the charge battery cellgroup.

Although the plurality of steps are shown in order in the flow chart ofFIG. 23, the order of performing the steps is not limited to the order.

According to the above embodiment, when an electric charge istransferred from the discharge battery cell group to the charge batterycell group, a structure where an electric charge from the dischargebattery cell group is temporarily stored, and the stored electric chargeis sent to the charge battery cell group is unnecessary, unlike in the acapacitor type circuit. Accordingly, the charge transfer efficiency perunit time can be increased. In addition, the switching circuit BT04 andthe switching circuit BT05 determine which battery cell in the dischargebattery cell group and the charge battery cell group to be connected tothe voltage transformer circuit.

Furthermore, the voltage transformer circuit BT07 converts thedischarging voltage applied to the terminal pair BT01 into the chargingvoltage based on the number of the battery cells BT09 included in thedischarge battery cell group and the number of the battery cells BT09included in the charge battery cell group, and applies the chargingvoltage to the terminal pair BT02. Thus, even when any battery cell BT09is selected as the discharge battery cell group and the charge batterycell group, an electric charge can be transferred without any problems.

Furthermore, the use of OS transistors as the transistor BT10 and thetransistor BT13 can reduce the amount of electric charge leaking fromthe battery cells BT09 which do not belong to the charge battery cellgroup or the discharge battery cell group. Accordingly, a decrease incapacity of the battery cells BT09 which do not contribute to chargingor discharging can be suppressed. In addition, the variation incharacteristics of the OS transistor due to heat is smaller than that ofan Si transistor. Accordingly, even when the temperature of the batterycells BT09 is increased, an operation such as turning on or off thetransistors in response to the control signals S1 and S2 can beperformed normally.

EMBODIMENT 6

Tn this embodiment, an example of an electronic device including thesecondary battery including the negative electrode described inEmbodiment 1 is described.

FIGS. 24A to 24F illustrate examples of electronic devices includingflexible secondary batteries. Examples of an electronic device includinga flexible secondary battery include television sets (also referred toas televisions or television receivers), monitors of computers or thelike, digital cameras or digital video cameras, digital photo frames,mobile phones (also referred to as cellular phones or mobile phonedevices), portable game machines, portable information terminals, audioreproducing devices, and large game machines such as pachinko machines.

In addition, a flexible secondary battery can be incorporated along acurved inside/outside wall surface of a house or a building or a curvedinterior/exterior surface of an automobile.

FIG. 24A illustrates an example of a mobile phone. A cellular phone 7400is provided with a display portion 7402 incorporated in a housing 7401,an operation button 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400includes a secondary battery 7407.

The mobile phone 7400 illustrated in FIG. 24B is bent. When the wholemobile phone 7400 is curved by external force, the secondary battery7407 included in the mobile phone 7400 is also curved. FIG. 24Cillustrates the curved secondary battery 7407. The secondary battery7407 is a thin secondary battery. The secondary battery 7407 is curvedand fixed.

FIG. 24D illustrates an example of a bangle display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102, anoperation button 7103, and a secondary battery 7104. FIG. 24Eillustrates the bent secondary battery 7104. When the curved secondarybattery 7104 is on a user's arm, the housing changes its form and thecurvature of a part or the whole of the secondary battery 7104 ischanged. Note that the radius of curvature of a curve at a point refersto the radius of the circular are that best approximates the curve atthat point. The reciprocal of the radius of curvature is curvature.Specifically, a part or the whole of the housing or the main surface ofthe secondary battery 7104 is changed in the range of radius ofcurvature from 40 mm to 150 mm. When the radius of curvature at the mainsurface of the secondary battery 7104 is 40 mm to 150 mm, thereliability can be kept high.

A flexile secondary battery can be provided with high space efficiencyin any of a variety of electronic devices. For example, in a stove 410illustrated in FIG. 24F, a module 411 is attached to a main body 412.The module 411 includes the secondary battery 200, a motor, a fan, anair outlet 411 a, and a thermoelectric generation device. In the stove410, after a fuel is injected through an opening 412 a and ignited,outside air can be sent through the air outlet 411 a to the inside ofthe stove 410 by rotating the motor and the fan which are included inthe module 411 using power of the secondary battery 200. In this manner,the stove 410 can have strong heating power because outside air can betaken into the inside of the stove 410 efficiently. In addition, cookingcan be performed on an upper grill 413 with thermal energy generated bythe combustion of fuel. The thermal energy is converted into power withthe thermoelectric generation device of the module 411, and thesecondary battery 200 is charged with the power. The power charged intothe secondary battery 200 can be output through an external terminal 411b.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

EMBODIMENT 7

In this embodiment, examples of electronic devices that can include thesecondary battery including the negative electrode described inEmbodiment 1 are described.

FIGS. 25A and 25B illustrate an example of a foldable tablet terminal. Atablet terminal 9600 illustrated in FIGS. 25A and 25B includes a housing9630 a, a housing 9630 b, a movable portion 9640 connecting the housings9630 a and 9630 b, a display portion 9631 provided with a displayportion 9631 a and a display portion 9631 b, a display mode switch 9626,a power switch 9627, a power saver switch 9625, a fastener 9629, and anoperation switch 9628. FIGS. 25A and 25B illustrate the tablet teiminal9600 opened and closed, respectively.

The tablet terminal 9600 includes a secondary battery 9635 inside thehousings 9630 a and 9630 b. The secondary battery 9635 is providedacross the housings 9630 a and 9630 b, passing through the movableportion 9640.

Part of the display portion 9631 a can be a touch panel region 9632 a,and data can be input by touching operation keys 9638 that aredisplayed. Note that FIG. 25A shows, as an example, that half of thearea of the display portion 9631 a has only a display function and theother half of the area has a touch panel function. However, thestructure of the display portion 9631 a is not limited to this, and allthe area of the display portion 9631 a may have a touch panel function.For example, all the area of the display portion 9631 a can displaykeyboard buttons and serve as a touch panel while the display portion9631 b can be used as a display screen.

As in the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a keyboard display switching button9639 displayed on the touch panel is touched with a finger, a stylus, orthe like, a keyboard can be displayed on the display portion 9631 b.

Touch input can be performed in the touch panel region 9632 a and thetouch panel region 9632 b at the same time.

The switch 9626 for switching a display mode allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. The power saver switch 9625 can control displayluminance in accordance with the amount of external light in use of thetablet terminal 9600, which is measured with an optical sensorincorporated in the tablet terminal 9600. In addition to the opticalsensor, other detecting devices such as sensors for determininginclination, such as a gyroscope or an acceleration sensor, may beincorporated in the tablet terminal.

Note that FIG. 25A shows an example in which the display portion 9631 aand the display portion 9631 b have the same display area; however, oneembodiment of the present invention is not limited and one of thedisplay portions may be different from the other display portion in sizeand display quality. For example, one of the display portions 9631 a and9631 b may display higher definition images than the other.

The tablet terminal is closed in FIG. 25B. The tablet terminal includesthe housing 9630, a solar cell 9633, and a charge and discharge controlcircuit 9634 including a DC-DC converter 9636. The secondary battery ofone embodiment of the present invention is used for the secondarybattery 9635.

The tablet terminal 9600 can be folded so that the housings 9630 a and9630 b overlap with each other when not in use. Thus, the displayportions 9631 a and 9631 b can be protected, which increases thedurability of the tablet terminal 9600. In addition, the secondarybattery 9635 of one embodiment of the present invention has flexibilityand can be repeatedly bent without a large decrease in charge anddischarge capacity. Thus, a highly reliable tablet terminal can beprovided.

The tablet terminal illustrated in FIGS. 25A and 25B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, or the time on the display portion, a touch-input function ofoperating or editing data displayed on the display portion by touchinput, a function of controlling processing by various kinds of software(programs), and the like.

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar cell 9633can be provided on one or both surfaces of the housing 9630 and thesecondary battery 9635 can be charged efficiently. When the secondarybattery of one embodiment of the present invention is used as thesecondary battery 9635, a tablet terminal can be used for a long periodbecause the deterioration of discharge capacity caused by repetition ofcharging and discharging can be suppressed.

The structure and the operation of the charge and discharge controlcircuit 9634 illustrated in FIG. 25B will be described with reference toa block diagram in FIG. 25C. The solar cell 9633, the secondary battery9635, the DC-DC converter 9636, a converter 9637, switches SW1 to SW3,and the display portion 9631 are illustrated in FIG. 25C, and thesecondary battery 9635, the DC-DC converter 9636, the converter 9637,and the switches SW1 to SW3 correspond to the charge and dischargecontrol circuit 9634 in FIG. 25B.

First, an example of operation in the case where power is generated bythe solar cell 9633 using external light is described. The voltage ofelectric power generated by the solar cell is raised or lowered by theDC-DC converter 9636 to a voltage for charging the secondary battery9635. When the display portion 9631 is operated with the power from thesolar cell 9633, the switch SW1 is turned on and the voltage of thepower is raised or lowered by the converter 9637 to a voltage needed foroperating the display portion 9631. In addition, when display on thedisplay portion 9631 is not performed, the switch SW1 is turned off andthe switch SW2 is turned on so that the secondary battery 9635 may becharged.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, one embodiment of the present invention isnot limited to this example. The secondary battery 9635 may be chargedusing another power generation means such as a piezoelectric element ora thermoelectric conversion element (Peltier element). For example, thesecondary battery 9635 may be charged with a non-contact powertransmission module capable of performing charging by transmitting andreceiving electric power wirelessly (without contact), or any of theother charge means used in combination.

The secondary battery including the negative electrode described inEmbodiment 1 can be provided in wearable devices illustrated in FIG. 26.

For example, the secondary battery can be provided in a glasses-typedevice 400 illustrated in FIG. 26. The glasses-type device 400 includesa frame 400 a and a display part 400 b. The secondary battery 200 isprovided in a temple of the frame 400 a having a curved shape, wherebythe glasses-type device 400 can have a well-balanced weight and can beused continuously for a long time.

The secondary battery can be provided in a headset-type device 401. Theheadset-type device 401 includes at least a microphone part 401 a, aflexible pipe 401 b, and an earphone part 401 c. The secondary battery200 can be provided in the flexible pipe 401 b and the earphone part 401c.

Furthermore, the secondary battery can be provided in a device 402 thatcan be attached directly to a body. The secondary battery 402 b isprovided in a thin housing 402 a of the device 402.

Furthermore, the secondary battery can be provided in a device 403 thatcan be attached to clothes. The secondary battery 403 b can be providedin a thin housing 403 a of the device 403.

Furthermore, the secondary battery can be provided in an armband device404. In the armband device 404, a display portion 404 b is provided overa main body 404 a and a secondary battery 404 c can be provided in themain body 404 a.

Furthermore, the secondary battery can be provided in a watch-typedevice 405. The watch-type device 405 includes a display portion 405 aand a belt portion 405 b, and the secondary battery 200 can be providedin the display portion 405 a or the belt portion 405 b.

Furthermore, the secondary battery can be provided in a belt-type device406. The belt-type device 406 includes a display portion 406 a and awireless power feeding and receiving portion 406 b, and the secondarybattery 200 can be provided inside the belt portion 406 a.

FIG. 27 illustrates examples of other electronic devices. In FIG. 27, adisplay device 8000 is an example of an electronic device using asecondary battery 8004 of one embodiment of the present invention.Specifically, the display device 8000 corresponds to a display devicefor TV broadcast reception and includes a housing 8001, a displayportion 8002, speaker portions 8003, the secondary battery 8004, and thelike. The secondary battery 8004 of one embodiment of the presentinvention is provided in the housing 8001. The display device 8000 canreceive power from a commercial power source. Alternatively, the displaydevice 8000 can use power stored in the secondary battery 8004. Thus,the display device 8000 can be operated with the use of the secondarybattery 8004 of one embodiment of the present invention as anuninterruptible power source even when power cannot be supplied from acommercial power source due to power failure or the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 8002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like besides TV broadcast reception.

In FIG. 27, an installation lighting device 8100 is an example of anelectronic device using a secondary battery 8103 of one embodiment ofthe present invention. Specifically, the installation lighting device8100 includes a housing 8101, a light source 8102, the secondary battery8103, and the like. Although FIG. 27 illustrates the case where thesecondary battery 8103 is provided in a ceiling 8104 on which thehousing 8101 and the light source 8102 are installed, the secondarybattery 8103 may be provided in the housing 8101. The installationlighting device 8100 can receive power from a commercial power source.Alternatively, the installation lighting device 8100 can use powerstored in the secondary battery 8103. Thus, the installation lightingdevice 8100 can be operated with the use of the secondary battery 8103of one embodiment of the present invention as an uninterruptible powersource even when power cannot be supplied from a commercial power sourcedue to power failure or the like.

Note that although the installation lighting device 8100 provided in theceiling 8104 is illustrated in FIG. 27 as an example, the secondarybattery of one embodiment of the present invention can be used as aninstallation lighting device provided in, for example, a wall 8105, afloor 8106, a window 8107, or the like other than the ceiling 8104.Alternatively, the secondary battery can be used in a tabletop lightingdevice or the like.

As the light source 8102, an artificial light source which emits lightartificially by using electric power can be used. Specifically, anincandescent lamp, a discharge lamp such as a fluorescent lamp, andlight-emitting elements such as an LED and an organic EL element aregiven as examples of the artificial light source.

In FIG. 27, an air conditioner including an indoor unit 8200 and anoutdoor unit 8204 is an example of an electronic device using asecondary battery 8203 of one embodiment of the present invention.Specifically, the indoor unit 8200 includes a housing 8201, an airoutlet 8202, the secondary battery 8203, and the like. Although FIG. 27illustrates the case where the secondary battery 8203 is provided in theindoor unit 8200, the secondary battery 8203 may be provided in theoutdoor unit 8204. Alternatively, the secondary batteries 8203 may beprovided in both the indoor unit 8200 and the outdoor unit 8204. The airconditioner can receive power from a commercial power source.Alternatively, the air conditioner can use power stored in the secondarybattery 8203. Particularly in the case where the secondary batteries8203 are provided in both the indoor unit 8200 and the outdoor unit8204, the air conditioner can be operated with the use of the secondarybattery 8203 of one embodiment of the present invention as anuninterruptible power source even when power cannot be supplied from acommercial power source due to power failure or the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 27 as an example, thesecondary battery of one embodiment of the present invention can be usedin an air conditioner in which the functions of an indoor unit and anoutdoor unit are integrated in one housing.

In FIG. 27, an electric refrigerator-freezer 8300 is an example of anelectronic device using a secondary battery 8304 of one embodiment ofthe present invention. Specifically, the electric refrigerator-freezer8300 includes a housing 8301, a door for a refrigerator 8302, a door fora freezer 8303, the secondary battery 8304, and the like. The secondarybattery 8304 is provided inside the housing 8301 in FIG. 27. Theelectric refrigerator-freezer 8300 can receive power from a commercialpower source. Alternatively, the electric refrigerator-freezer 8300 canuse power stored in the secondary battery 8304. Thus, the electricrefrigerator-freezer 8300 can be operated with the use of the secondarybattery 8304 of one embodiment of the present invention as anuninterruptible power source even when power cannot be supplied from acommercial power source due to power failure or the like.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

EMBODIMENT 8

In this embodiment, examples of vehicles including the secondary batteryincluding the negative electrode described in Embodiment 1 aredescribed.

The use of secondary batteries in vehicles enables production ofnext-generation clean energy vehicles such as hybrid electric vehicles(HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles(PHEVs).

FIGS. 28A and 28B each illustrate an example of a vehicle using oneembodiment of the present invention. An automobile 8400 illustrated inFIG. 28A is an electric vehicle which runs on the power of the electricmotor. Alternatively, the automobile 8400 is a hybrid electric vehiclecapable of driving using either the electric motor or the engine asappropriate. One embodiment of the present invention achieves ahigh-mileage vehicle. The automobile 8400 includes the secondarybattery. The secondary battery is used not only to drive the electricmotor, but also to supply electric power to a light-emitting device suchas a headlight 8401 or a room light (not illustrated).

The secondary battery can also supply electric power to a display deviceof a speedometer, a tachometer, or the like included in the automobile8400. Furthermore, the secondary battery can supply electric power to asemiconductor device included in the automobile 8400, such as anavigation system.

FIG. 28B illustrates an automobile 8500 including the secondary battery.The automobile 8500 can be charged when the secondary battery issupplied with electric power through external charging equipment by aplug-in system, a contactless power feeding system, or the like. In FIG.28B, the power storage device included in the automobile 8500 is chargedwith the use of a ground-based charging apparatus 8021 through a cable8022. In charging, a given method such as CHAdeMO (registered trademark)or Combined Charging System may be referred to for a charging method,the standard of a connector, or the like as appropriate. The chargingapparatus 8021 may be a charging station provided in a commerce facilityor a power source in a house. With the use of a plug-in technique, thesecondary battery included in the automobile 8500 can be charged bybeing supplied with electric power from outside, for example. Thecharging can be performed by converting AC electric power into DCelectric power through a converter such as an AC-DC converter.

Further, although not illustrated, the vehicle may include a powerreceiving device so as to be charged by being supplied with electricpower from an above-ground power transmitting device in a contactlessmanner. In the case of the contactless power supply system, by fittingthe power transmitting device in a road or an exterior wall, chargingcan be performed not only when the electric vehicle is stopped but alsowhen driven. In addition, the contactless power supply system may beutilized to perform transmission/reception between vehicles. A solarcell may be provided in the exterior of the automobile to charge thesecondary battery when the automobile stops or moves. To supply electricpower in such a contactless manner, an electromagnetic induction methodor a magnetic resonance method can be used.

According to one embodiment of the present invention, the secondarybattery can have improved cycle characteristics and reliability.Furthermore, according to one embodiment of the present invention, thesecondary battery itself can be made compact and lightweight as a resultof improved characteristics of the secondary battery. The compact andlightweight secondary battery contributes to a reduction in the weightof a vehicle, and thus increases the driving radius. Furthermore, thesecondary battery included in the vehicle can be used as a power sourcefor supplying electric power to products other than the vehicle. In sucha case, the use of a commercial power source can be avoided at peak timeof electric power demand.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

EXAMPLE 1

In this example, description is given of the comparison results aftercharging and discharging of a negative electrode having a plurality ofprojections and depressions provided in the negative electrode currentcollector and the negative electrode active material layer and thenegative electrode which does not have projections and depressionsprovided in the negative electrode current collector and the negativeelectrode active material layer.

(Sample A)

First, a negative electrode having a plurality of projections anddepressions provided in a negative electrode current collector and anegative electrode active material layer is formed as a sample A, and asecondary battery which incorporates the negative electrode was chargedand discharged while pressure was applied to the secondary battery.

The negative electrode of the sample A was formed in the followingmanner. First, 18-μm-thick copper foil was used as a negative electrodecurrent collector. SiO with a particle size of about 5 μm was used asthe negative electrode active material, and acetylene black (AB) andpolyimide (PI) as a conductive additive and a binder were mixed thereto.The ratio of the SiO, AB and PI in the mixture was 80:5:15 (weightratio). One surface of the negative electrode current collector wascoated with the mixture, whereby the negative electrode active materiallayer was formed.

After the negative electrode active material layer was formed on thenegative electrode current collector, a plurality of projections anddepressions were formed parallel to each other with a space of about 1mm by using a cutter knife. FIG. 29 shows a negative electrode in whicha plurality of projections and depressions were formed in the negativeelectrode current collector and the negative electrode active materiallayer. FIG. 29A shows a surface of the negative electrode currentcollector on which the negative electrode active material layer wasformed, FIG. 29B shows a surface of the negative electrode currentcollector on which the negative electrode active material layer was notformed.

A secondary battery was fabricated using the negative electrode formedthrough the above step. Other materials of the secondary battery aredescribed below.

Aluminum was used for a positive electrode current collector. A materialrepresented by a composition formula Li_(1.68)Mn_(0.8062)Ni_(0.318)O₃was used as a positive electrode active material, and acetylene black(AB) and PVDF as a conductive additive and a binder were mixed thereto.The ratio of Li_(1.68)Mn_(0.8062)Ni_(0.318)O₃, AB, and PVDF in themixture was 90:5:5 (weight ratio). The capacity of the positiveelectrode with respect to the capacity of the negative electrode was85%.

As a separator, polypropylene was used.

An electrolyte solution was formed by dissolving 1.2 mol/L of LiPF₆ inan organic solvent in which EC, DEC, and EMC were mixed at a weightratio of 3:6:1, and adding 0.5 weight % of propanesultone (PS) and 0.5weight % of vinylene carbonate (VC) thereto as an additive.

As an exterior body, an aluminum laminate film was used. As the aluminumlaminate film, 40-μm-thick aluminum one surface of which was adhered to25 μm-thick oriented nylon with a 3-μm-thick dry laminate layer and theother surface of which was provided with 45-μm-thick polypropylene wasused.

The secondary battery which was manufactured using the above materialswas charged and discharged while pressure was applied to the secondarybattery.

Pressure of 10 MPa was applied in a thickness direction of the secondarybattery (i.e., the direction parallel to the shortest of the length,width, and height of the exterior body of the secondary battery).

The charging and discharging was performed by CC charging anddischarging (charge termination voltage is 4.6 V, discharge terminationvoltage is 1.5 V).

(Sample B)

Next, a negative electrode was fabricated as a sample B in a mannersimilar to that of the sample A, except that projections and depressionswere not formed in a negative electrode current collector and a negativeelectrode active material layer. Then, the negative electrodeincorporated in a secondary battery was charged and discharged whilepressure was applied to the secondary battery in manner similar to thatof the sample A.

(Sample C)

Next, a negative electrode was fabricated as a sample C in a mannersimilar to that of the sample B. Then, the negative electrodeincorporated in a secondary battery was charged and discharged in amanner to similar to that of the sample B, except that pressure was notapplied during charging and discharging.

FIGS. 30A to 30C are X-ray CT images of negative electrodes of thesample A, the sample B, and the sample C manufactured in theabove-described manner after being charged and discharged. FIG. 30A isan image of the sample A, FIG. 30B is an image of the sample B, and FIG.30C is an image of the sample C.

Large wrinkles were generated in the negative electrode of the sample Cwhich had no projections and depressions in the negative electrodecurrent collector and the negative electrode active material layer andwas not pressurized during charging and discharging. The difference inheight between the peak and the valley of the wrinkle was approximately200 μm to 300 μm, and the volume of the secondary battery was increased.Furthermore, a shadow showing accumulation of a gas generated bydecomposition of an electrolyte solution was observed in deep wrinkles.

Wrinkles in the negative electrode current collector were suppressed butclearly observed in the sample B which was pressurized during chargingand discharging.

On the contrary, wrinkles were not observed in the sample A which hasprojections and depressions in the negative electrode current collectorand the negative electrode active material layer and was pressurizedduring charging and discharging.

Furthermore, the negative electrodes of the sample A, the sample B, andthe sample C were taken out from the secondary batteries and observed.FIG. 31A is a photograph of the negative electrode of the sample A, FIG.31B is a photograph of the negative electrode of the sample B, and FIG.31C is a photograph of the negative electrode of the sample C.

Portions where the negative electrode active material layer wasseparated along wrinkles were observed in the sample C which had noprojections and depressions in the negative electrode current collectorand the negative electrode active material layer and was not pressurizedduring charging and discharging. In addition, the color of the negativeelectrode active material layer was abnormal.

The defects observed in the sample C were reduced in the sample B whichwas pressurized during charging and discharging.

The separation of the negative electrode active material layer and theabnormal color thereof were not observed in the sample A which hasprojections and depressions in the negative electrode current collectorand the negative electrode active material layer and was pressurizedduring charging and discharging.

The above-described result reveals that forming a plurality ofprojections and depressions in the negative electrode current collectorand the negative electrode active material layer can suppressdeformation of the negative electrode by absorbing expansion of thenegative electrode active material due to charging.

EXAMPLE 2

In this example, initial charge and discharge characteristics of thesample A and the sample B in Example 1 (i.e., the negative electrodehaving a plurality of projections and depressions in the negativeelectrode current collector and the negative electrode active materiallayer, and the negative electrode having no projections and depressionsin the negative electrode current collector and the negative electrodeactive material layer) are described.

FIG. 32 shows a graph of initial charge and discharge characteristics ofthe sample A and the sample B described in Example 1. The solid lineshows the charge and discharge characteristics of the sample A, and thedotted line shows those of the sample B. The upward curve is a chargecurve, and the downward curve is a discharge curve.

The result of FIG. 32 reveals that a negative electrode having aplurality of projections and depressions in the negative electrodecurrent collector and the negative electrode active material layer hashigher discharge capacity than the negative electrode not having aplurality of projections and depressions in the negative electrodecurrent collector and the negative electrode active material layer.

This application is based on Japanese Patent Application serial no.2014-217229 filed with Japan Patent Office on Oct. 24, 2014, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a secondary batterycomprising the step of: performing a first charging and dischargingwhile pressure is applied in a thickness direction of the secondarybattery, wherein the secondary battery includes: a negative electrode; apositive electrode; an electrolyte solution; and a separator, whereinthe negative electrode includes a negative electrode current collectorand a negative electrode active material layer, wherein the negativeelectrode active material layer comprises SiO_(y) (2>y>0), wherein thenegative electrode active material layer includes a first projection, afirst depression, a second projection and a second depression, whereinthe negative electrode current collector includes a third projection, athird depression, a fourth projection and a fourth depression, whereinthe first projection, the first depression, the third projection and thethird depression overlap with each other, and wherein the secondprojection, the second depression, the fourth projection and the fourthdepression overlap with each other.
 2. The method according to claim 1,wherein the first projection, the first depression, the secondprojection, the second depression, the third projection, the thirddepression, the fourth projection and the fourth depression are formedusing a cutter knife.
 3. The method according to claim 1, wherein thepressure is applied at 10 MPa.
 4. The method according to claim 1,wherein the negative electrode current collector is exposed at a bottomof a depression of the first projection and the first depression.
 5. Themethod according to claim 1, wherein the first depression, the seconddepression, the third depression, and the fourth depression has a linearshape.
 6. The method according to claim I. wherein the SiO_(y) comprisesan amorphous silicon.
 7. The method according to claim 1, furthercomprising a graphene on a surface of the negative electrode activematerial layer.